The fatty acid compositions of the serum cholesterol esters, phospholipids and triglycerides were determined in 28 insulin-dependent diabetic children and 13 healthy controls. The diabetic children were on a regulated diet and the disease was under good control. The relative contents of linoleic and arachidonic acids were higher in the serum lipids of the diabetics than in the controls as could be expected from the dietary advice given. However, the degree of diabetic control was not significantly correlated to the fatty acid content of any lipid fraction. The serum concentration of apolipoprotein (apo) A-I was directly correlated to the content of polyunsaturated fat and the ratio between polyunsaturated and saturated fatty acids in the triglycerides. The apo A-II concentration was significantly correlated to the ratio between homo-gamma-linolenic and arachidonic acid in all serum lipid esters. No similar relationships were seen among the healthy controls. The different relationships between serum apolipoprotein concentrations and the fatty acid composition in serum in diabetics and controls is compatible with the hypothesis that not only the quantity but also the quality of the serum lipoproteins are different in these two categories of children.