Previous investigations of in vitro fertilization (IVF) cycles stimulated with gonadotropins have shown that it is possible to predict IVF outcome on the basis of the pattern of estradiol (E2) production in the terminal phase of follicular growth. This analysis looked at endocrine and ultrasound parameters in the late follicular phase of 320 patients in their first cycle of IVF. All cycles were stimulated by an association of gonadotropin-releasing hormone agonists (GnRH-a) and human menopausal gonadotropins (hMG). The pregnancy rate (PR) was not related to the E2 pattern in the 5 days before ovulation induction and was 33% and 39% even when E2 values fell during hMG administration. The PR was independent of the E2 level and the number of follicles greater than 14 mm on the day of human chorionic gonadotropin administration (day 0). The incidence of multiple pregnancy increased when E2 levels were greater than 1,000 pg/mL. It is concluded that follicular phase parameters used for cycle cancellation in hMG-stimulated IVF cycles cannot be extrapolated to GnRH-a/hMG cycles.