How common are best mode rejections in patent examination?

This page is an FAQ based on guidance from the Manual of Patent Examining Procedure. It is provided as guidance, with links to the ground truth sources. This is information only: it is not legal advice.

Best mode rejections are extremely rare in patent examination, particularly in ex parte prosecution. The MPEP 2165.03 explicitly states:

“It is extremely rare that a best mode rejection properly would be made in ex parte prosecution. The information that is necessary to form the basis for a rejection based on the failure to set forth the best mode is rarely accessible to the examiner, but is generally uncovered during inter partes proceedings.”

This rarity is due to the difficulty in obtaining evidence that the inventor knew of a better mode at the time of filing but failed to disclose it. Such information is typically not available to examiners during standard patent prosecution.