To assess secular trends in serum lipid levels in Finnish children and young adults, the authors examined a total of 3,517, 2,769, 2,392, 352, and 880 subjects who had complete data on serum lipids in 1980, 1983, 1986, 1989, and 1992, respectively, in a longitudinal follow-up study. Trend analyses were carried out among subjects aged 15 (n = 1,835) or 18 (n = 1,562) years to exclude the confounding effect of age on the study variables. Data on obesity, physical activity, smoking, and alcohol use were available from each study year, and data on diet were available for the study years 1980, 1986, and 1992. Between 1980 and 1992, mean total cholesterol levels decreased from 4.88 to 4.47 mmol/liter (from 189 to 173 mg/dl), and low density lipoprotein cholesterol levels decreased from 3.06 to 2.85 mmol/liter (from 119 to 110 mg/dl). The mean high density lipoprotein cholesterol levels decreased by 19%, from 1.43 to 1.15 mmol/liter (55.2 to 44.6 mg/dl). During 1986-1992, triglyceride levels increased by 15%, from 0.88 to 1.01 mmol/liter (78.2 to 89.9 mg/dl). During 1980-1992, body mass index values increased from 20.8 to 21.8 kg/m2, parallel to increases in skinfold thickness. In the diet, the ratio of polyunsaturated to saturated fatty acids increased from 0.26 to 0.39. Alcohol and oral contraceptive use became more frequent, and the subjects tended to become less physically active. In conclusion, a change in the lipid profile in Finnish adolescents aged 15 and 18 years and young adults during 1980-1992 was observed, characterized by a decrease in low density lipoprotein cholesterol and high density lipoprotein cholesterol levels and an increase in triglyceride level. Possible determinants for these changes include alterations in diet and a trend toward increased obesity.