Objective: To compare the efficacy and tolerability of two recombinant human FSH (r-hFSH) preparations, follitropin-alpha (Gonal-F; Ares Serono, Geneva, Switzerland) and follitropin-beta (Puregon;Organon, Oss, the Netherlands), for superovulation in patients undergoing IVF-ET. Design: Randomized, parallel-group, assessor-blind, single-center trial. Setting: Outpatient tertiary referral center for assisted reproductive techniques. Patient(s): Forty-four infertile women undergoing IVF-ET. Intervention(s): After down-regulation with buserelin acetate, patients were randomized to receive follitropin-alpha or follitropin-beta, 150 IU/d for 6 days; after that, dosages were adjusted according to the ovarian response. Main Outcome Measure(s): Cumulative dose of r-hFSH; duration of r-hFSPI treatment; number of follicles of greater than or equal to 11 mm and of 14 mm on day 7 of r-hFSPI treatment and on the day of hCG administration; number of oocytes retrieved; number of viable embryos; and number of pregnancies (biochemical, ectopic, miscarried) and clinical pregnancies. Result(s): There were no statistically significant differences in any efficacy measures between the two preparations. The incidence of systemic adverse events was comparable in the two groups. Local reactions at the inject-ion site were significantly more common and more severe with follitropin-beta than with follitropin-alpha Conclusion(s): Follitropin-alpha and follitropin-beta have comparable efficacy in patients undergoing IVF-ET. Fertil Steril(R) 2000;73:114-6. (C)1999 by American Society for Reproductive Medicine.