Three hundred and ninety-six patients were evaluated for primary and secondary infertility between December 1976 and May 1979 at a large referral center. Timed late luteal endometrial biopsies were routinely obtained as part of the work-up and were repeated for confirmation if subsequent menses did not occur within 2 days of the expected date. If both biopsies were abnormal, a diagnosis of luteal phase defect (LPD) was made and patients were treated with vaginal progesterone suppositories for a minimum of 6 months. LPD was discovered in 32 of 396 patients (8.1%); among those patients whose infertility was not complicated by other abnormalities, 9 of 13 conceived (70%) and 7 of 13 carried to term (54%). These data suggest an incidence higher than generally recognized and a very encouraging response to replacement therapy.