Between 1973 and 1975, 16 patients evaluated for infertility at Duke University Medical Center were diagnosed as having luteal phase deficiency. A majority had had prior infertility surveys, and the average duration of their infertility exceeded 2 years. The diagnosis was suspected after study of basal body temperature charts and menstrual patterns in more than 80% of the patients. This diagnosis was established by timed endometrial biopsy. The primary method of therapy was supplementation of the luteal phase with progesterone vaginal suppositories. The pregnancy rate after therapy was 50% and pregnancy occurred after a mean of five treatment cycles. The minimal follow-up of patients who failed to conceive was 8 months. To date, the majority of these pregnancies have been completed without complication and the remainder are progressing satisfactorily. Two additional patients developed luteal phase deficiency while taking clomiphene citrate and became pregnant with progesterone supplementation.