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 < 0.001), the total cholesterol:high-density lipoprotein cholesterol ratio decreased significantly from 3.8+/-0.3 to 3.3+/-0.2 at 6 months (P < 0.001) and a similar trend was observed in serum triglyceride levels during metformin treatment. In the oral contraceptive group, serum levels of total cholesterol increased from 4.9+/-0.3 to 5.4+/-0.3 mmol/l (P < 0.05), high-density lipoprotein cholesterol increased from 1.2+/-0.1 to 1.5+/-0.1 mmol/l (P < 0.001), the total cholesterol:high-density lipoprotein cholesterol ratio decreased from 4.6+/-0.4 to 3.7+/-0.2 (P < 0.001) and triglycerides increased from 1.3+/-0.1 to 1.9+/-0.2 mmol/l at 6 months of treatment (P < 0.001). Serum low-density lipoprotein cholesterol levels remained unchanged during both treatments. Milder but similar changes in the subgroups of obese and non-obese women were observed during both treatments. Moreover, in the whole-study population both systolic (P = 0.02) and diastolic (P = 0.05) blood pressures decreased over the 6 months of metformin treatment. CONCLUSIONS In women with PCOS, metformin treatment had beneficial effects on lipid profile and blood pressure, and therefore it could be useful in the prevention of cardiovascular complications in these women.