Five regularly menstruating women of proven fertility, with normal prolactin and thyroid function studies, underwent a total of 39 endometrial biopsies (EMBs). The slides were dated in blinded fashion, and the cycle date determined by considering the date of the next menstrual period as day 28 and counting backward. Using a 2-day or greater lag in endometrial maturity to define a luteal phase defect (LPD), the incidence of single and sequential out-of-phase EMBs was 51.4% and 26.7%, respectively. Using a 3-day or greater lag to define a LPD, the incidence of single and sequential out-of-phase EMBs was 31.4% and 6.6%, respectively. These incidences in normal, fertile women are as high as the rates quoted for infertile populations, and call into question the standard criteria for defining this condition and evaluating therapies to correct it.