High density lipoprotein cholesterol (HDLC) was measured in the population surveys carried out in Finland in 1982 and 1987. The mean (+/- SD) of HDLC for men aged 25-64 years was 1.23 +/- 0.32 mmol/l in 1982 and 1.30 +/- 0.33 mmol/l in 1987, the increase being 5.7%. Respective values in women were 1.46 +/- 0.33 mmol/l and 1.58 +/- 0.35 mmol/l, an increase of 8.2%. The increase was significantly greater in women than in men (P less than 0.001). The prevalence of low HDLC (less than or equal to 0.9 mmol/l) fell from 13.5 to 9.6% in men and from 3.2 to 1.4% in women. Factors associated with low HDLC were the history of ischemic heart disease (IHD), diabetes and hypertension in both sexes and the positive family history of IHD in women. Subjects with low HDLC also had higher body mass index (BMI), waist-hip ratio and serum total cholesterol (TC) concentration than subjects with normal HDLC. Changes in BMI, waist-hip ratio, alcohol intake, leisure time physical activity, prevalence of smoking or in the dietary variables recorded, did not explain the increase in HDLC.