Purpose: The effects of premature luteinization of ovarian follicles as detected by elevated progesterone values on the day of human chorionic gonadotropin induction of ovulation were evaluated in 38 consecutive gamete intrafallopian transfer (GIFT) retrieval cycles. Materials and Methods: All patients received leuprolide acetate beginning in the midluteal phase of their prior menstrual cycle, followed by gonadotropin stimulation of folliculogenesis. At least four oocytes were transferred in each cycle. Results: No significant differences in gonadotropin dosage, total number of days of gonadotropins, age, number of prior pregnancies, years of infertility since last pregnancy, total number of eggs retrieved, mature residual oocytes, fertilization of mature residual oocytes, or primary etiology of infertility were observed between groups; however; estradiol concentrations were significantly higher in the group with elevated progesterone values (2573 +/- 216 pg/ml) compared to the lower progesterone group (1925 +/- 202 pg/ml, P = 0.035) and the total number of oocytes transferred was greater in the high progesterone group (7.5 +/- 0.5) vs the low progesterone group (6.3 +/- 0.3, P < 0.038). P-4 concentrations less than or equal to 0.8 ng/ml were associated with significantly higher pregnancy rates (11/19; 57.9%) compared to progesterone concentrations >0.8 ng/ml (5/19, 26.3%; P = 0.050). Conclusions: Premature luteinization may occur in luteal leuprolide acetate-down-regulated patients and progesterone values >0.8 ng/ml are associated with significantly lower pregnancy rates in GIFT cycles.