To investigate whether or not there is any relation between endometrial morphology and subsequent pregnancy rate, 47 couples with unexplained infertility were followed up for 3 years after they had had an endometrial biopsy. Each woman had an LH-timed endometrial biopsy performed in the luteal phase of the cycle. The biopsy was dated chronologically according to the luteinizing hormone (LH) surge and histologically using morphometric criteria. None of the women received any form of treatment in the endometrial biopsy cycle. Of 47 women with unexplained infertility, 36 (76.6%) (Group I) had 'in phase' endometrial development and 11 (23.4%) (Group II) had retarded endometrium. Women with normal endometrial development had a higher pregnancy rate than women with retarded endometrial development (50% vs. 9%; P less than 0.02). The fecundability during treatment cycles was higher in Group I than in Group II (0.051 vs. 0.008; P less than 0.05). In women with 'in phase' endometrium, treatment increased the monthly probability of conception (0.051 vs. 0.006; P less than 0.001). A precisely timed endometrial biopsy should be considered as part of the investigations for women with unexplained reproductive failure, to help determine prognosis.