We have examined the efficacy of highly purified follicle stimulating hormone (FSH-HP) for controlled ovarian stimulation in our in-vitro fertilization (IVF) programme, and compared the results obtained with this preparation with those using human menopausal gonadotrophin (HMG) in 15 patients who had received treatment with both FSH-HP and HMG in consecutive cycles (n = 39). No differences were found in the duration of stimulation, which was 13.9 days (HMG) as compared with 14.3 days (FSH-HP). However, in the FSH-HP-treated cycles we found a striking difference in the rise of serum oestradiol, which was significantly lower than in HMG-treated cycles (2953 +/- 938 pmol/l as compared with 6349 +/- 3683 pmol/l on the day before ovum retrieval). Number and size of follicles were similar in the two groups, as were oocyte characteristics. Increase in endometrial thickness at two days prior to ovum retrieval was slightly higher after HMG. The results indicate that in combination with a long gonadotrophin-releasing hormone agonist (GnRHa) protocol, pure FSH is sufficient for adequate follicle recruitment and growth. However, since FSH-HP resulted in markedly reduced concentrations of serum oestradiol as compared to HMG cycles, IVF programmes using repeated oestradiol measurements to decide the day of ovum retrieval must take this into consideration in order not to prolong the stimulation unnecessarily.