To correlate luteal estradiol (E2) levels with pregnancy outcome, 36 consecutive conceptions resulting from gamete intrafallopian transfer in gonadotropin releasing hormone agonist/human menopausal gonadotropin (GnRH-a/hMG) cycles were analyzed. GnRH-a was initiated during the preceding luteal phase. HMG was adjusted individually. Human chorionic gonadotropin (hCG), 5,000 IU, was administered when E2 was > 500 pg/mL and the leading follicle > 17 mm (day 0). The luteal phase was supported by (1) hCG, 1,500 IU in three doses from day 5 and (2) progesterone (P) from day 7. E2 and P levels were analyzed in three groups of patients: normally progressing pregnancy (NPP), abortion (AB) and preclinical abortion (PAB). No significant differences in mean E2 levels were seen between the groups from day 0 through day 5 after hCG. Midluteal E2 levels were significantly different between the groups (P < .05). Late luteal E2 values were significantly higher for NPP than for either AB or PAB (P