OBJECTIVE: To evaluate the initial versus early pattern of estradiol (E2) change after administration of a gonadotropin-releasing hormone agonist (GnRH-a), i.e., the GnRH-a stimulation test versus E2 pattern, respectively, as predictors of ovarian response and pregnancy in in vitro fertilization (IVF) patients stimulated with a flare-up protocol. DESIGN: Prospective study in a consecutive group of patients. SETTING: Tertiary care, institutional setting. PATIENTS: Two hundred twenty-eight patients entered and completed the study. The only patients excluded from study were those anticipated to have polycystic ovarian disease, those with a single ovary, or those with an ovarian cyst(s). INTERVENTIONS: Patients were stimulated with a GnRH-a flare-up protocol beginning on menstrual day 2. MAIN OUTCOME: Evaluation of the GnRH-a stimulation test and the E2 pattern as predictors of the number of mature oocytes retrieved and pregnancy. RESULTS: The GnRH-a stimulation test but not the E2 pattern was predictive of the number of mature oocytes retrieved (r = 0.53, P less than 1 X 10(-5) and pregnancy (chi 2 = 8.5, P = 0.04). The E2 pattern was predictive of the duration and number of ampules of gonadotropin required for stimulation. CONCLUSION: The GnRH-a stimulation test is a sensitive predictor of performance in the flare-up IVF cycle.