ABSTRACT. Fatty acid compositions of serum cholesteryl esters (CE) were analysed with gas chromatography from a total of 1,348 Finnish children. The study was a part of a comprehensive survey of atherosclerosis precursors among children, and included 3-, 6-, 9-, 12-, 1.5, and 18-year-old children and adolescents from five urban and twelve rural communities in Finland. In all age groups and both sexes, the mean percentages of linoleate (CE-18:2) were lower and those of saturated and monounsaturated fatty acids higher in eastern rural areas than elsewhere. Eastern rural children also had significantly higher proportions of ω3 polyunsaturated fatty acids in their serum CE's. The percentage of serum CE-18:2 parellels the PIS values obtained by a dietary survey from the same populations. The reason for higher percentages of the ω3 fatty acids in rural communities cannot be explained by diet, and remains unclear.