Serum total cholesterol and high-density lipoprotein (HDL) cholesterol concentrations were measured in 1122 men and 1147 women aged between 25 and 64 years during the first Belfast MONICA survey, and the results subjected to multiple regression analysis. In both men and women, total cholesterol increased with age. Although HDL-cholesterol showed little variation with age, the values were considerably higher in women than men. Total cholesterol increased with body mass index while HDL-cholesterol decreased, and these findings persisted after adjustment for age. Regular exercise was associated with higher HDL-cholesterol values, even after adjustment for age and body mass index. Among men and women who abstained from alcohol, lower values of HDL-cholesterol were observed. In both sexes, cigarette smoking was associated with significant increases in total cholesterol values and decreases in HDL-cholesterol values, though some of these findings became apparent only after adjustment for other relevant factors. Perhaps surprisingly, a measure of health knowledge showed no association with blood lipid concentrations.