OBJECTIVE: To assess the performance of Fertinex (urofollitropin; Serono Laboratories, Norwell, MA) in stimulating the development of multiple follicles and initiating subsequent pregnancy in patients undergoing controlled ovarian hyperstimulation (COH) for IVF. DESIGN: Prospective observational study. SETTING: Private assisted reproductive technology (ART) center. PATIENT(S): Ninety-three women who underwent an IVF cycle of COH in which Fertinex was used as the sole gonadotropin. INTERVENTION(S): The COH protocol included Fertinex with leuprolide acetate down-regulation in the late luteal phase or the early follicular phase. Embryo transfer was performed after 3 days of culture. MAIN OUTCOME MEASURE(S): Stimulation parameters, embryologic data, and pregnancy rates (PRs). RESULT(S): Ninety-three patients underwent 97 cycles of COH, with a cancellation rate of 13.4%. The clinical PR was 42% per initiation, 49% per retrieval, and 51% per transfer. Shorter stimulation periods, lower estradiol levels either per follicle punctured or per oocyte retrieved, and slower embryo development were observed. Patients responded to Fertinex in three distinct stimulation patterns: escalating response with a peak estradiol level of < or =3,000 pg/mL, escalating response with a peak estradiol level of >3,000 pg/mL, or escalating response characterized by a drop in the estradiol level or a plateau before hCG administration. All three responses had similar PRs. CONCLUSION(S): Fertinex can be used successfully as the sole gonadotropin for COH in ART without compromising high PRs. Traditional estradiol response curves or cancellation criteria may not apply when Fertinex is used as the sole gonadotropin for COH in ART.