Background: There have been numerous double-blind, randomised, dose-finding studies of the gonadotrophin-releasing hormone (Gn-RH) antagonist Ganirelix (Org 37462) [2] and the Gn-RH antagonist Cetrorelix [4]. Methods: We performed a clinical trial with 26 patients undergoing ovarian stimulation with the recombinant human follicle sti­mulating hormone (FSH) by applying rec. FSH in a flexible daily dose, depending on the estradiol levels of the patient, from day 3 of the cycle onwards in a controlled ovarian hyperstimulation (COH) protocol. A single dose of 0.25 mg was injected daily sub­cutaneously from day 7 of the cycle onwards until the day of HCG application. The recombinant FSH dose was adjusted according to ovarian response. Fifteen patients were treated for IVF and nine for ICSI. In two patients egg retrieval was not performed. Estra­diol and LH levels were measured on the day of HCG application. Results: The mean number of recovered oocytes in the 24 patients was 5.5. A fertilisation rate of 64% was achieved in the IVF group and 69% in the ICSI group. In Germany a maximum of three embryos per patient is permitted to be transferred. Pregnancy occurred in ten patients: 5 IVF patients and 5 ICSI patients. In this small group of IVF patients a 33% pregnancy rate per follicular puncture and a 50% pregnancy rate per embryo transfer resulted. The ICSI group showed a 55% pregnancy rate per follicular puncture and a 62% pregnancy rate per embryo transfer. Conclusion: Within an IVF/ET, ICSI programme the Gn-RH-antagonist Cetrorelix in combination with rec. FSH gave optimal pregnancy results.