A multicentre study on atherosclerosis precursors in young Finns aged three to 18 years was started in 1980 (3596 subjects) serum lipid concentrations (cholesterol, HDL (high density lipoprotein) cholesterol and triglycerides) were determined (n = 3554) and the apolipoproteins A-I and B measured (n = 1355). Two follow-up studies were carried out in 1983 (n = 2851) and 1986 (n = 2489), when HDL-subfractions (HDL-2-cholesterol and HDL-3-cholesterol) were also determined. Apolipoproteins A-I and B were measured again in 1986 (n = 1202). Serum total cholesterol concentration has fallen by about 1% annually during the 1980's from 5.07 mmol/l (1980) to 4.79 mmol/l (1986) in 9- to 18-year old children and adolescents. Mean values of serum triglycerides have slightly increased during the follow-up from 0.79 mmol/l to 0.84 mmol/l, respectively. In children and young adults (3-24 years) the mean cholesterol concentration was highest at the age of six and lowest during puberty. Concentrations of serum cholesterol, LDL (low density lipoprotein) cholesterol apoprotein B and triglycerides were higher in eastern than in western Finland in 1980 and 1983, but these differences were smaller in 1986, with the exception of serum triglycerides. Both in 1983 and in 1986 HDL-2-cholesterol was lower in the west than in the east, whereas HDL-3-cholesterol was higher in the former. The favourable changes in lipid levels should be reflected in future morbidity and mortality rates from coronary heart disease in Finland.