Young people with insulin-dependent diabetes mellitus (IDDM) (n = 104, 7 to 19 years old) were classified into groups of good (I), fair (II), and poor (III) diabetic control and compared to 22 healthy controls of same origin and age range. Triglycerides (TG) were significantly elevated with respect to controls only in group III, and total cholesterol was increased by over 40% (p < 0.01) mainly due to low density lipoprotein-cholesterol (LDL-C). Apolipoprotein B (apo B) was over 40% higher in groups II and III. TG, LDL-C, and apo B concentrations all positively correlated with HbA1c levels. Neither HDL-C nor apolipoprotein AI (apo AI) were affected by glycaemic control, but phospholipids were increased in all three subgroups of IDDM subjects (p < 0.001). Over the course of 6 months' treatment, TG, apo B, and LDL-C varied in parallel with HbA1c, confirming a tight association with the quality of glycaemic control. Apolipoprotein AIV (apo AIV) was higher in all groups and closely related to glycaemic control (r = +0.43, p < 0.001), independent of TG or HDL. The specific and sensitive response of apo AIV to insulin deficiency suggests a direct effect at the level of intestinal synthesis.