Previous reports agree that estrogen and estrogen-containing contraceptives increase serum triglyceride levels of normal women, but disagree on their effect on serum cholesterol levels. Since obesity is often accompanied by hyperinsulinemia and since hyperinsulinemia may participate in the production of hypertriglyceridemia, we investigated the effect of oral contraceptives onthe serum lipids of obese women. Serum triglycerides and cholesterol were measured before and after 3 months administration of the contraceptives. The mean triglyceride level increased 23%in the obese and 21% in the normal women. The mean cholesterol level increased 6% (P < 0.05) in the obese and did not change in the normal women. The increase in cholesterol occurred mostly in those with initial levels less than 225 mg/100 ml; in those with initial levels above 250 mg/100 ml the level usually decreased with treatment. The serum lipid changes were not related to the serum insulin levels.