The luteal phase was evaluated by endometrial biopsy (EMB) 11 to 13 days after administration of human chorionic gonadotropin (hCG) or initiation of the luteinizing hormone (LH) surge (hCG/LH) in 25 stimulated patients not undergoing embryo transfer (ET). Luteal phase defect (LPD) was defined as an EMB greater than 2 days out-of-phase when compared with the onset of subsequent menses. Nineteen women (76%) had LPDs ranging from 3 to 7 days. Mean estradiol (E2) on day of hCG/LH was significantly (P less than 0.01) higher (1709 +/- 997) in patients with in-phase EMBs than mean E2 levels in two groups of patients with out-of-phase EMBs (687 +/- 251, 941 +/- 366, respectively). These findings are further evidence of the importance of optimal follicular development to subsequent endometrial maturation.