We studied endometrial luteal phase in specimens from 660 biopsies done in 300 patients from our infertility clinic. A minimum of two (240 women) or three (60 women) endometrial biopsy specimens from separate cycles were taken regardless of the previous histologic findings in all patients. Statistical analysis of results by the McNemar and the Cochran Q tests for the significance of changes leads us to conclude that a minimum of two, and even three, endometrial biopsy specimens are needed for diagnosis of luteal phase deficiency.