To identify abnormalities of serum lipoprotein composition and concentration that were specific to insulin-dependent diabetes mellitus (IDDM), the procedure of discontinuous gradient ultracentrifugation was employed to isolate lipoprotein fractions in 44 patients with IDDM, 24 nondiabetic subjects with similar lipid and lipoprotein concentrations, and 19 healthy normocholesterolemic (less than 5.2 mmol/l [less than 200 mg/dl]) subjects. The mass concentration of low density lipoprotein (LDL) was greater in IDDM than in both control groups. The free cholesterol to phospholipid ratio in large very low density lipoprotein (VLDL) was greatest in IDDM in comparison with both of the other groups. The contribution of triglyceride to total large VLDL mass was greater, whereas that of phospholipids was lower, in IDDM than in the dyslipidemic nondiabetic group. Protein concentration was reduced and phospholipid increased in small VLDL in IDDM in comparison with both control groups, and the contribution from protein to lipoprotein mass was least in IDDM. Similarly in intermediate density lipoprotein (IDL), the protein concentration and its contribution to overall mass was also lower in IDDM than in either control group, but by contrast, the phospholipid content was increased. The cholesteryl ester to protein ratio was highest in both small VLDL and IDL in IDDM in comparison with both control groups, whereas the free cholesterol to phospholipid ratio in IDL was least in IDDM. In LDL, total cholesterol and triglyceride concentrations were greatest and the contribution from protein to lipoprotein mass was least in IDDM in comparison with both control groups. The LDL free cholesterol to phospholipid ratio was greater in IDDM than in dyslipidemic control subjects.(ABSTRACT TRUNCATED AT 250 WORDS)