The luteal phase in a group of women (n = 49) with unexplained infertility was evaluated by improved methods, including the use of the LH surge for precise timing of measurements, quantitative techniques for histological dating of endometrial biopsy and determination of the integrated progesterone concentration in the luteal phase. Overall, 10 out of 49 (20%) women were considered to have a retarded endometrium. The prevalence of retarded endometrium in women with primary infertility (10/37, 27%) was found to be significantly higher (P less than 0.05) than in women with secondary infertility (0/12, 0%). Women with retarded endometrial development had a significantly (P less than 0.01) lower integrated P concentration than those with normal endometrial development; however, of the 10 cases with retarded endometrium, only four were associated with an integrated P concentration below the normal range, whereas the remaining six were associated with an integrated P concentration within the normal reference range, suggesting that an abnormal response of the endometrium is a likely cause of abnormality in these latter cases. There was no difference in age, duration of infertility, follicular phase length, magnitude of the oestrogen surge, magnitude of the LH surge, maximum follicular diameter and luteal phase length between women with a normal or defective luteal phase, defined either by endometrial morphology or integrated progesterone concentration.