Objective: To analyze the luteal phase of six patients undergoing controlled ovarian hyperstimulation (COH) with hMG and a new GnRH antagonist, Cetrorelix, without receiving luteal phase supplementation. Design: Phase II study involving the first six patients who did not receive luteal phase support. Setting: Tertiary referral center. Patient(s): Six healthy women undergoing COH for assisted reproductive techniques. Intervention(s): Oocyte retrieval was performed 36 hours after hCG administration, followed by embryo transfer 2 days later. No luteal phase supplementation was given. Main Outcome Measure(s): Serum E-2, progesterone, LH, and FSH concentrations were measured. Result(s): The length of the luteal phase was less than or equal to 12 days in three of the six patients. One of the patients in whom the luteal phase was >12 days had a low serum progesterone concentration (2.9 ng/mL) on day 10. Serum LH concentrations decreased after the preovulatory hCG injection in all patients. However, a progressive increase in LH was observed after day 7, reaching normal values. Conclusion(s): Corpus luteum function seems to be impaired in cycles that are stimulated with hMG and the GnRH antagonist Cetrorelix. (Fertil Sterile 1998;70:357-9; (C) 1998 by American Society for Reproductive Medicine.).