A group of 17 patients with suspected luteal phase deficiency was treated with tamoxifen. Tamoxifen therapy was found to lengthen the luteal phase in all patients and resulted in pregnancy in 6 of 17 patients. The integrated luteal phase progesterone (P) concentration in the nontreatment cycle of seven patients was significantly lower (P less than 0.01) than that of five normal women. Therapy with tamoxifen increased the P concentration to 186.0 +/- 24.4 ng/ml/cycle (mean +/- standard error of the mean), i.e., twice that of the control cycle. The mean estradiol (E2) concentration at the midcycle peak was about twice that observed during the nontreatment cycle. The glycogen content of the endometrial tissue at the midluteal phase in the tamoxifen cycle was significantly higher (P less than 0.025) than that of endometrial tissue in the nontreatment cycle, indicating improvement of the endometrial function.