This multicentre, randomized study was performed to assess the efficacy and safety of 0.25 mg ganirelix (Orgalutran, Antagon) treatment, using triptorelin (Decapeptyl) in a long protocol as a reference treatment. In total, 236 subjects were randomized to treatment with ganirelix (0.25 mg, s.c.) and 119 to triptorelin (0.1 mg, s.c.) treatment (treatment ratio 2:1). Treatment with ganirelix started on day 6 of stimulation, whereas treatment with triptorelin started on menstrual cycle day 21 to 24 of the previous cycle (i.e. the midluteal phase). The ganirelix regimen was on average 17 days shorter (9 versus 26 days) compared to the triptorelin regimen. The median total dose of recombinant FSH (Puregon) used was 450 IU less (1350 versus 1800 IU) in the ganirelix protocol. The initial follicular growth was faster and, consequently, oestradiol concentrations were higher in the ganirelix group. On the day of human chorionic gonadotrophin (HCG), the mean number of follicles > or = 11 mm was 10.1 and 10.7 and the median serum oestradiol concentration was 1090 and 1370 pg/ml in the ganirelix and triptorelin groups respectively. Per attempt, 7.9 and 9.6 oocytes (mean) were retrieved in the ganirelix and triptorelin groups respectively. The fertilization rates (64.0% ganirelix and 64.9% triptorelin) and the mean number of good quality embryos (2.7 and 2.9) were comparable in both treatment groups. The implantation rate was identical (22.9%). The ongoing pregnancy rate per attempt was 31.0 and 33.9% in the ganirelix and triptorelin groups respectively. The ganirelix regimen showed an improved local tolerance in that the percentage of subjects with at least one local skin reaction was 2-fold lower than in the triptorelin group (11.9 versus 24.1%). Taking all data together, it may be concluded that ganirelix offers a new treatment regimen in ovarian stimulation that is short, safe and well-tolerated, optimizing convenience for the patient.