Serum lipoprotein lipid and apolipoprotein concentrations were determined in 27 diabetic children (5-18 years old) and 13 matched healthy controls. The serum cholesterol concentrations in the diabetics were slightly higher than in the controls (P less than 0.05) due to a significantly higher level of the high density lipoprotein cholesterol (P less than 0.01). Also the serum concentration of apolipoprotein A-I, the major protein constituent of the high density lipoprotein fraction, was higher in the diabetic children (P = 0.05). There were no significant differences between the groups with regard to the serum triglyceride concentrations or the apolipoprotein C-II and C-III concentrations. Neither the lipoprotein lipid nor the apolipoprotein levels were significantly correlated with variables related to the degree of regulation of the diabetic disease. No obvious explanation, based on the present data, can be given for the increased high density lipoprotein cholesterol concentrations in insulin-treated diabetics in comparison with the healthy children. It is possible, however, that the increased high density lipoprotein cholesterol concentration may be caused by an increased level of insulin in the circulation of insulin-treated diabetic children.