New gonadotrophin-releasing hormone (GnRH) antagonists, which allow suppression of luteinizing hormone (LH) surges, have recently become available. We compared in this study the results of a single administration of 3 versus 2 mg Cetrorelix in 65 patients undergoing ovarian stimulation and in-vitro fertilization (IVF). The GnRH antagonist (Cetrorelix) was non-randomly administered at a dose of 3 mg (34 patients) or 2 mg (32 patients) on day 8 of the stimulation cycle. In the case of slow follicular development, the injection was delayed until oestradiol reached 400 pg/ml. No difference was observed in the decrease in LH and in oestradiol secretion between the 3 and the 2 mg groups, but the LH secretion was suppressed for a shorter time in the 2 mg group. No LH surge was observed in the 3 mg group, while one surge (3%) and one significant rise in LH were observed in the 2 mg group. No significant difference was observed in IVF results in the two groups of patients. This study demonstrates that a single injection of 3 or 2 mg successfully prevents LH surges for at least 3 days in all the patients treated. The LH rises in the 2 mg group led us to choose the 3 mg dose as a safer dose in our single administration protocol.