MPEP 2138.06
"Reasonable Diligence"

Ninth Edition of the MPEP, Revision 07.2022, Last Revised in February 2023

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2138.06    "Reasonable Diligence" [R-07.2022]

[Editor Note: This MPEP section has limited applicability to applications subject to examination under the first inventor to file (FITF) provisions of the AIA as set forth in 35 U.S.C. 100 (note). See MPEP § 2159 et seq. to determine whether an application is subject to examination under the FITF provisions, MPEP § 2159.03 for the conditions under which this section applies to an AIA application, and MPEP § 2150 et seq. for examination of applications subject to those provisions.]

The diligence of pre-AIA 35 U.S.C. 102(g) relates to reasonable "attorney-diligence" and "engineering-diligence" (Keizer v. Bradley, 270 F.2d 396, 397, 123 USPQ 215, 216 (CCPA 1959)), which does not require that "an inventor or his attorney … drop all other work and concentrate on the particular invention involved…." Emery v. Ronden, 188 USPQ 264, 268 (Bd. Pat. Inter. 1974).


The critical period for diligence for a first conceiver but second reducer begins not at the time of conception of the first conceiver but just prior to the entry in the field of the party who was first to reduce to practice and continues until the first conceiver reduces to practice. Hull v. Davenport, 90 F.2d 103, 105, 33 USPQ 506, 508 (CCPA 1937) ("lack of diligence from the time of conception to the time immediately preceding the conception date of the second conceiver is not regarded as of importance except as it may have a bearing upon his subsequent acts"). What serves as the entry date into the field of a first reducer is dependent upon what is being relied on by the first reducer, e.g., conception plus reasonable diligence to reduction to practice (Fritsch v. Lin, 21 USPQ2d 1731, 1734 (Bd. Pat. App. & Inter. 1991), Emery v. Ronden, 188 USPQ 264, 268 (Bd. Pat. Inter. 1974)); an actual reduction to practice or a constructive reduction to practice by the filing of either a U.S. application (Rebstock v. Flouret, 191 USPQ 342, 345 (Bd. Pat. Inter. 1975)) or reliance upon priority under 35 U.S.C. 119 of a foreign application (Justus v. Appenzeller, 177 USPQ 332, 339 (Bd. Pat. Inter. 1971) (chain of priorities under 35 U.S.C. 119 and 120, priority under 35 U.S.C. 119 denied for failure to supply certified copy of the foreign application during pendency of the application that was filed within the twelve month period prescribed by 35 U.S.C. 119 )).


"A patent owner need not prove the inventor continuously exercised reasonable diligence throughout the critical period; it must show there was reasonably continuous diligence." Perfect Surgical Techniques, Inc. v. Olympus Am., Inc., 841 F.3d 1004, 1009, 120 USPQ2d 1605, 1609 (Fed. Cir. 2016) (emphasis in original) (citing Tyco Healthcare Grp. v. Ethicon EndoSurgery, Inc., 774 F.3d 968, 975, 112 USPQ2d 1979 (Fed. Cir. 2014) and Monsanto Co. v. Mycogen Plant Sci., Inc., 261 F.3d 1356, 1370, 59 USPQ2d 1930, 1939 (Fed. Cir. 2001)).

An inventor must account for the entire period during which diligence is required. Gould v. Schawlow, 363 F.2d 908, 919, 150 USPQ 634, 643 (CCPA 1966) (Merely stating that there were no weeks or months that the invention was not worked on is not enough.); In re Harry, 333 F.2d 920, 923, 142 USPQ 164, 166 (CCPA 1964) (statement that the subject matter "was diligently reduced to practice" is not a showing but a mere pleading). A 2-day period lacking activity has been held to be fatal. In re Mulder, 716 F.2d 1542, 1545, 219 USPQ 189, 193 (Fed. Cir. 1983) (37 CFR 1.131 issue); Fitzgerald v. Arbib, 268 F.2d 763, 766, 122 USPQ 530, 532 (CCPA 1959) (Less than 1 month of inactivity during the critical period shows a lack of diligence. Efforts to exploit an invention commercially do not constitute diligence in reducing it to practice. An actual reduction to practice in the case of a design for a three-dimensional article requires that it should be embodied in some structure other than a mere drawing.); Kendall v. Searles, 173 F.2d 986, 993, 81 USPQ 363, 369 (CCPA 1949) (Evidence of diligence must be specific as to dates and facts.).

The period during which diligence is required must be accounted for by either affirmative acts or acceptable excuses. Rebstock v. Flouret, 191 USPQ 342, 345 (Bd. Pat. Inter. 1975); Rieser v. Williams, 225 F.2d 419, 423, 118 USPQ 96, 100 (CCPA 1958) (Being last to reduce to practice, party cannot prevail unless he has shown that he was first to conceive and that he exercised reasonable diligence during the critical period from just prior to opponent’s entry into the field); Griffith v. Kanamaru, 816 F.2d 624, 2 USPQ2d 1361 (Fed. Cir. 1987) (Court generally reviewed cases on excuses for inactivity including vacation extended by ill health and daily job demands, and held lack of university funding and personnel are not acceptable excuses.); Litchfield v. Eigen, 535 F.2d 72, 190 USPQ 113 (CCPA 1976) (budgetary limits and availability of animals for testing not sufficiently described); Morway v. Bondi, 203 F.2d 741, 749, 97 USPQ 318, 323 (CCPA 1953) (voluntarily laying aside inventive concept in pursuit of other projects is generally not an acceptable excuse although there may be circumstances creating exceptions); Anderson v. Crowther, 152 USPQ 504, 512 (Bd. Pat. Inter. 1965) (preparation of routine periodic reports covering all accomplishments of the laboratory insufficient to show diligence); Wu v. Jucker, 167 USPQ 467, 472-73 (Bd. Pat. Inter. 1968) (applicant improperly allowed test data sheets to accumulate to a sufficient amount to justify interfering with equipment then in use on another project); Tucker v. Natta, 171 USPQ 494,498 (Bd. Pat. Inter. 1971) ("[a]ctivity directed toward the reduction to practice of a genus does not establish, prima facie, diligence toward the reduction to practice of a species embraced by said genus"); Justus v. Appenzeller, 177 USPQ 332, 340-1 (Bd. Pat. Inter. 1971) (Although it is possible that patentee could have reduced the invention to practice in a shorter time by relying on stock items rather than by designing a particular piece of hardware, patentee exercised reasonable diligence to secure the required hardware to actually reduce the invention to practice. "[I]n deciding the question of diligence it is immaterial that the inventor may not have taken the expeditious course….").

Examiners should weigh the body of evidence to make the determination of whether there was reasonably continuous diligence for the entire critical period. This determination should be made with the purpose of the diligence requirement in mind, which "is to assure that, in light of the evidence as a whole, ‘the invention as not abandoned or unreasonably delayed.’" Perfect Surgical Techniques, Inc. v. Olympus Am., Inc. (Perfect Surgical), 841 F.3d 1004, 1009, 120 USPQ2d 1605, 1609 (Fed. Cir. 2016) (citations omitted). In Perfect Surgical, the Federal Circuit stated:

Under this standard, an inventor is not required to work on reducing his invention to practice every day during the critical period. … And periods of inactivity within the critical period do not automatically vanquish a patent owner's claim of reasonable diligence…. In determining whether an invention antedates another, the point of the diligence analysis is not to scour the patent owner's corroborating evidence in search of intervals of time where the patent owner has failed to substantiate some sort of activity. It is to assure that, in light of the evidence as a whole, "the invention was not abandoned or unreasonably delayed."... That an inventor overseeing a study did not record its progress on a daily, weekly, or even monthly basis does not mean the inventor necessarily abandoned his invention or unreasonably delayed it. The same logic applies to the preparation of a patent application: the absence of evidence that an inventor and his attorney revised or discussed the application on a daily basis is alone insufficient to determine that the invention was abandoned or unreasonably delayed. One must weigh the collection of evidence over the entire critical period to make such a determination.

Our decision in In re Mulder, 716 F.2d 1542, 1542-46 [219 USPQ 189] (Fed. Cir. 1983), does not instruct otherwise. The Board cites In re Mulder for the proposition that "[e]ven a short period of unexplained inactivity may be sufficient to defeat a claim of diligence." …. In In re Mulder, a competing reference was published just days before the patent at issue was constructively reduced to practice. …. The patent owner was tasked with showing reasonable diligence during a critical period lasting only two days.... But the patent owner did not produce any evidence of diligence during the critical period.... Nor could it point to any activity during the months between the drafting of the application and the start of the critical period.... Although the critical period spanned just two days, we declined to excuse the patent owner's complete lack of evidence.... In re Mulder does not hold that an inventor's inactivity during a portion of the critical period can, without more, destroy a patent owner's claim of diligence.

Perfect Surgical, 841 F.3d at 1009, 120 USPQ2d at 1609 (citations omitted).


The work relied upon to show reasonable diligence must be directly related to the reduction to practice of the invention in issue. Naber v. Cricchi, 567 F.2d 382, 384, 196 USPQ 294, 296 (CCPA 1977), cert. denied, 439 U.S. 826 (1978). See also Scott v. Koyama, 281 F.3d 1243, 1248-49, 61 USPQ2d 1856, 1859 (Fed. Cir. 2002) (Activities directed at building a plant to practice the claimed process of producing tetrafluoroethane on a large scale constituted efforts toward actual reduction to practice, and thus were evidence of diligence. The court distinguished cases where diligence was not found because inventors either discontinued development or failed to complete the invention while pursuing financing or other commercial activity.); In re Jolley, 308 F.3d 1317, 1326-27, 64 USPQ2d 1901, 1908-09 (Fed. Cir. 2002) (diligence found based on research and procurement activities related to the subject matter of the interference count). "[U]nder some circumstances an inventor should also be able to rely on work on closely related inventions as support for diligence toward the reduction to practice on an invention in issue." Ginos v. Nedelec, 220 USPQ 831, 836 (Bd. Pat. Inter. 1983) (work on other closely related compounds that were considered to be part of the same invention and which were included as part of a grandparent application). "The work relied upon must be directed to attaining a reduction to practice of the subject matter of the counts. It is not sufficient that the activity relied on concerns related subject matter." Gunn v. Bosch, 181 USPQ 758, 761 (Bd. Pat. Inter. 1973) (An actual reduction to practice of the invention at issue which occurred when the inventor was working on a different invention "was fortuitous, and not the result of a continuous intent or effort to reduce to practice the invention here in issue. Such fortuitousness is inconsistent with the exercise of diligence toward reduction to practice of that invention." 181 USPQ at 761. Furthermore, evidence drawn towards work on improvement of samples or specimens generally already in use at the time of conception that are but one element of the oscillator circuit of the count does not show diligence towards the construction and testing of the overall combination.); Broos v. Barton, 142 F.2d 690, 691, 61 USPQ 447, 448 (CCPA 1944) (preparation of application in U.S. for foreign filing constitutes diligence); De Solms v. Schoenwald, 15 USPQ2d 1507 (Bd. Pat. App. & Inter. 1990) (principles of diligence must be given to inventor’s circumstances including skill and time; requirement of corroboration applies only to testimony of inventor); Huelster v. Reiter, 168 F.2d 542, 78 USPQ 82 (CCPA 1948) (an inventor who was not able to make an actual reduction to practice of the invention, must also show why constructive reduction to practice by filing an application was not possible).


The diligence of an attorney in preparing and filing patent application inures to the benefit of the inventor. Conception was established at least as early as the date a draft of a patent application was finished by a patent attorney on behalf of the inventor. Conception is less a matter of signature than it is one of disclosure. An attorney does not prepare a patent application on behalf of particular named persons, but on behalf of the true inventive entity. Six days to execute and file application is acceptable. Haskell v. Coleburne, 671 F.2d 1362, 213 USPQ 192, 195 (CCPA 1982). See also Bey v. Kollonitsch, 806 F.2d 1024, 231 USPQ 967 (Fed. Cir. 1986) (Reasonable diligence is all that is required of the attorney. Reasonable diligence is established if the attorney worked reasonably hard on the application during the continuous critical period. If the attorney has a reasonable backlog of unrelated cases which the attorney takes up in chronological order and carries out expeditiously, that is sufficient. Work on a related case(s) that contributed substantially to the ultimate preparation of an application can be credited as diligence.). In "the preparation" of a patent application, "the absence of evidence that an inventor and his attorney revised or discussed the application on a daily basis is alone insufficient to determine that the invention was abandoned or unreasonably delayed." Perfect Surgical Techniques, Inc. v. Olympus Am., Inc., 841 F.3d 1004, 1009, 120 USPQ2d 1605, 1609 (Fed. Cir. 2016).


"[I]t is of no moment that the end of that period [for diligence] is fixed by a constructive, rather than an actual, reduction to practice." Justus v. Appenzeller, 177 USPQ 332, 340-41 (Bd. Pat. Inter. 1971).