High density lipoprotein cholesterol (HDL chol) levels were measured in about 1000 forty-year-old residents of the town of Leiden, The Netherlands. Mean levels (+/- S.D.) in women were 48.0 +/- 11.0 mg/dl; in men 42.5 +/- 10.7 mg/dl. In March mean levels in women were about 5 mg/dl lower than in June (P less than 0.001); in men there was a similar difference between mean levels in April and June (P less than 0.002). HDL chol levels were negatively related to smoking habits, relative body weight, and in women to use of oral contraceptives. Weak positive and negative correlations between HDL chol and serum cholesterol, respectively, were found in subgroups with high and low HDL chol levels, respectively.