Hyperinsulinaemic insulin resistance is commonly associated with hyperandrogenaemia, and menstrual dysfunction. The aim of this study was to examine the effects of the insulin sensitizing drug, metformin, on ovarian function, follicular growth, and ovulation rate in obese women with oligomenorrhoea. Twenty obese subjects with oligomenorrhoea [polycystic ovarian syndrome; (PCOS)] were observed longitudinally for 3 weeks prior to and for 8 weeks during treatment with metformin (850 mg twice per day). Fifteen patients completed the study. The frequency of ovulation was significantly higher during treatment than before treatment (P = 0.003). A significant decline in both testosterone and luteinizing hormone concentrations was recorded within 1 week of commencing treatment. Patients with elevated pretreatment testosterone concentrations showed the most marked increase in ovulation rate (P < 0.005), and significant reductions in circulating testosterone from 1.02 to 0.54 ng/ml (P < 0.005) after only 1 week of treatment. However, the sub-group with raised fasting insulin showed less marked changes, and the sub-group with normal testosterone concentrations showed no effect of treatment. Metformin had a rapid effect upon the abnormal ovarian function in hyperandrogenic women with PCOS, correcting the disordered ovarian steroid metabolism and ovulation rate; however, there appeared to be no effect in cases where the circulating androgen concentration was normal.