Objective: To compare the following IVF protocols: a short protocol and long protocols starting on day 2 or day 21. All protocols used recombinant FSH (rFSH) for ovarian stimulation.Design: Prospective, randomized study.Setting: An assisted reproduction unit at a university center.Patient(s): Patients undergoing IVF (n = 150) were treated according to one of the three treatment protocols allocated by a random method.Intervention(s): All patients used buserelin acetate and rFSH. Patients began the long protocol on day 2 (group 1) or day 21 (group 2) of the cycle. Patients in group 3 began the short protocol on day 2 of the cycle.Main Outcome Measure(s): Ovarian response, implantation rates, and pregnancy rates (PRs).Result(s): Patients treated with the short protocol (group 3) were stimulated in a shorter time, required less rFSH, and achieved higher E-2 levels. The numbers of follicles, oocytes, and embryos were similar in all three groups. The implantation rates were similar in groups 1 and 2 (11.8% and 13.3%, respectively), and both were higher than in group 3 (4.1%). Clinical PRs per oocytes retrieved were 19.6%, 18.6% and 8.3% in groups 1, 2, and 3, respectively.Conclusion(s): The implantation rate was significantly higher with the long protocols than with the short protocol, with a tendency for a higher PR as well. (Fertil Steril(R) 2000;73:908 -12. (C) 2000 by American Society for Reproductive Medicine).