In a survey of risk factors for coronary heart-disease (C.H.D.) in a Leiden population aged 40-41 years, mean serum-high-density-lipoprotein (H.D.L.)-cholesterol was significantly higher in 447 women (47.94 mg/dl) than in 471 men (42.40 mg/dl). No association was found between serum-H.D.L.-cholesterol and systolic or diastolic blood-pressure, obesity, or electrocardiographic changes. Cigarette smoking with use of oral contraceptives were strongly associated with reduced serum-H.D.L.-cholesterol. The difference in mean serum-H.D.L.-cholesterol concentrations between women who were on oral contraceptives and those who were not was independent of the effect of smoking. The finding of a low mean serum-H.D.L.-cholesterol concentration in pill users who smoke (i.e., similar to that in men of the same age; 43.0 mg/dl and 42.4 mg/dl in women and men, respectively) is disturbing since low serum-H.D.L.-cholesterol is a major C.H.D. risk factor and because of the reported increase in mortality from circulatory diseases in women using oral contraceptives.