The triglyceride and cholesterol content of total, very-low-, intermediate-, low-, and high-density lipoproteins, and of apolipoproteins (apo) Al, All, B, Cll, Clll, and E were determined in plasma from 107 patients with clinically well-controlled diabetes and from 66 age- and weight-matched healthy normal subjects. The diabetic patients were separated into two groups: those with insulin-dependent diabetes mellitus (IDDM, type 1, n = 24) and those with non-insulin-dependent diabetes mellitus (NIDDM, type 2, n = 83). The latter group contained two subgroups: those treated by diet (type 2d, n = 42) or by insulin (type 2i, n = 41). High-density lipoprotein cholesterol was increased in IDDM patients, and decreased in NIDDM patients relative to control subjects. Mean apo Al values in IDDM patients were higher than in their respective controls and in NIDDM patients. Concentrations of apo B, Clll, and E were higher in all diabetic patients than in the healthy controls, but those of apo Cll did not differ statistically between diabetics and nondiabetics. Although total plasma cholesterol and triglyceride concentrations were apparently near normal values in patients with good glycemic control, we found a persistent increase of intermediate-density lipoproteins (remnants) in all the diabetic groups studied. This factor may be related to the perceived increased cardiovascular risk in these individuals.