The concentrations of apolipoproteins A-I and B were determined in 1,341 3- to 18-year-old children and adolescents from five urban and 12 rural communities. The analyses were made with radial immunodiffusion. The mean concentrations (+/- S.D.) of apo A-I and apo B were 152 +/- 25 and 94 +/- 22 mg/100 ml, respectively. 3-year-old children had the highest apo B levels which then decreased with advancing age in both sexes. Boys tended to have lower levels of apo B than girls. Apo A-I concentration was significantly higher in the 9- and 12-year-old boys than in the other age groups but showed no age-bound trend in girls. The apo A-I to apo B ratio increased with age in both sexes. The concentration of apo A-I was significantly lower, and that of apo B higher, in children living in eastern Finland in comparison with those from the western part of the country. This difference and a higher HDL-cholesterol to apo A-I ratio in both sexes in eastern Finland may be associated with the regional differences in the prevalence of coronary heart disease in this country.