A multicentre study on atherosclerosis precursors in Finnish children and adolescents was carried out in five urban and 12 rural areas in the autumn of 1980 and spring of 1981. Serum lipids, i.e. cholesterol (TC), HDL-cholesterol (HDL-C) and triglyceride (TG) concentrations were determined and LDL-cholesterol (LDL-C) was calculated in 630 newborns and 3,596 children aged 3, 6, 9, 12, 15 and 18 years. In the newborns the mean serum TC concentration was 1.50 mmol/l, and the ratio of HDL-C to TC was 0.44. Newborn boys had lower mean TC, HDL-C and LDL-C values than the girls. In 3 to 18-year-old children the mean TC, LDL-C and HDL-C concentrations were 4.83 mmol/l, 3.09 mmol/l and 1.38 mmol/l, respectively. During puberty, TC mean values decreased, more so in boys. The serum levels of HDL-C also decreased, especially in boys, and after passing puberty boys had lower mean HDL-C levels than girls (1.26 vs. 1.39 mmol/l, p less than 0.001). The HDL-C/TC ratio was similar in all age groups (0.29). The mean TG value increased with age, being 0.88 mmol/l at the age of 18 yr. During sexual maturation, TG levels increased, more clearly in boys. There were no regional differences in serum lipid concentrations in the newborns, but in 3- to 18-year-old children the mean TC, LDL-C and TG values were lower in western than in eastern Finland. Mean TC and LDL-C values were lower in urban than in rural areas, but there was no difference in TG concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)