The adverse effect of raised luteinizing hormone (LH) concentrations on reproductive outcome suggests that exogenous LH administration for ovarian stimulation may not be desirable. The aim of this study was to compare the clinical pregnancy rates between follicle stimulating hormone (FSH) and human menopausal gonadotrophin (HMG) used in in-vitro fertilization (IVF) cycles. A total of 232 infertile patients, with a mean duration of infertility of 67.1 +/- 32.9 months, were selected for IVF (female age < 38 years, FSH < 15 IU/l, and total motile sperm count > 5 x 10(6)). A short (flare-up) protocol with daily leuprolide acetate was followed randomly from day 3 with FSH (n = 115) or human menopausal gonadotrophin (HMG; n = 117), at an initial dose of two ampoules per day. A maximum of three embryos was transferred, and the luteal phase was supported with four doses of HCG (2500 IU). No differences were observed between the two groups in any of the cycle response variables except fertilization rates per oocyte and per patient, both of which were significantly higher with FSH. Clinical pregnancy rates per cycle initiated, per oocyte retrieval and per embryo transfer were 19.1, 21.0 and 22.7% respectively for FSH, and 12.0, 12.8 and 15.4% respectively for HMG. Whilst these differences were not statistically significant, the results of this interim analysis suggest that HMG may be associated with a lower clinical pregnancy rate than FSH.