Thirty-one insulin-dependent diabetic children were studied (11 boys and 20 girls; 22 whites, 6 mulattoes and 3 Negroes; age-range: 8-14 years; mean age 11.7 years). According to the quality of their metabolic control the children were divided into 3 groups: Group 1, children in good metabolic control; Group 2, children in poor metabolic control but without ketosis; Group 3, children in severe ketoacidosis; 15 normal children (Group 4) served as controls. No hypercholesterolemia was found. As to blood sugar and serum triglyceride levels, significant differences were found between the control group and the diabetic groups as well as between the diabetic groups. When evaluating the result of HDL-cholesterol determinations we found a significant difference between the control group and Groups 2 and 3, as well as between diabetics in good control (Group 1) compared to Groups 2 and 3. We also found a correlation in diabetics in good control between blood sugar values and high HDL-cholesterol levels, and in diabetics in poor control between high blood sugar values and low HDL-cholesterol levels. No correlation was found between HDL-cholesterol and triglycerides in diabetes in poor metabolic control. In view of these findings the importance of reaching an optimal metabolic control in insulin-dependent children is emphasized, and the role of normoglycemia in the prevention of atherosclerosis is stressed.