OBJECTIVE: Women with polycystic ovary syndrome (PCOS) exhibit risk factors for cardiovascular diseases such as abdominal obesity, insulin resistance and dyslipidemia. Insulin sensitizers, especially metformin, have been shown to improve these metabolic disturbances, but there are only a few studies on their effects on serum lipids in polycystic ovary syndrome. METHODS: Thirty-five women with PCOS (18 obese and 17 non-obese) were randomized to 6-month treatments with metformin or ethinyl estradiol-cyproterone acetate oral contraceptive pills. RESULTS: In the whole-study population (non-obese and obese women) serum levels of high-density lipoprotein cholesterol increased from 1.4+/-0.2 to 1.6+/-0.1 mmol/l (means +/-S.E. throughout) at 3 and 6 months (P