Continuous administration of gonadotrophin-releasing hormone analogues (GnRHa) in patients stimulated for the purpose of IVF might have a direct effect on the ovary. We have evaluated the IVF outcome of patients treated with Buserelin and subsequently with a combination of follicle-stimulating hormone (FSH) and human menopausal gonadotrophin (HMG). Patients were divided into three groups according to the number of oocytes obtained by transvaginal ultrasound-guided follicular aspiration: group 1 (n = 35), in which 1-5 oocytes were retrieved; group 2 (n = 30), in whom 6-10 oocytes were obtained; group 3 (n = 32), in whom greater than or equal to 11 oocytes were collected. Only couples with normal semen samples at oocyte retrieval were included in this study. The dose of Buserelin employed was not different between groups. However, the amount of FSH/HMG necessary to reach an optimal response significantly (P less than 0.01) decreased as follicular development increased. The quality of the oocytes obtained was evaluated based on the appearance of the oocyte-corona-cumulus complex, fertilization rate, morphological appearance of the embryos, and implantation rate. The fertilization rate was significantly (P less than 0.01) decreased in group 3 (57.2%) in comparison with groups 1 (77.1%) and 2 (74.2%). There was no significant difference between the groups in the quality of the embryos obtained or the quality of those replaced into the uterus. The implantation rate per embryo transferred was significantly (P less than 0.05) higher in group 1 (16.5%) in comparison with groups 2 (6.6%) and 3 (8.2%).(ABSTRACT TRUNCATED AT 250 WORDS)