To assess the usefulness of the late luteal phase endometrial biopsy in infertility, we evaluated a total of 1492 biopsies performed in 1055 patients. Of these women, 699 underwent one biopsy during spontaneous ovulatory cycles, 288 had two, 57 had three, nine had four, and five biopsies were done in two patients. As controls we included 45 fertile women who were requesting contraception. We analysed histological dating of the endometrium and its abnormality rates in first and successive biopsy specimens, as well as the association of the pregnancy outcome with the endometrial patterns and treatment for luteal phase deficiency (LPD). Our results show firstly that diagnosis of LPD in both infertile and fertile women represents only a chance event; secondly, histological endometrial adequacy or inadequacy in the cycle of conception or in previous cycles is not related to the outcome of pregnancy in infertile patients. Finally, treatment of LPD does not improve pregnancy outcome in infertile women. Thus, luteal phase evaluation by histological dating of the endometrium is not worthwhile.