Hyperlipoproteinemia is found in only 25 to 50% of persons with diabetes. However, carbohydrate, lipid and lipoprotein metabolism are so closely related that one would expect some alterations in lipoprotein metabolism in nearly all diabetic subjects. Whether the metabolism of lipoproteins in normolipemic diabetic patients was normal, and whether the metabolism of lipoproteins in subjects with hyperlipoproteinemia and diabetes differed from those without diabetes, was investigated. As a first approach to these questions, the levels of lipoprotein lipids and apolipoproteins were determined in 97 diabetics and 72 nondiabetics, and lipoprotein compositions were computed. Among those with diabetes, 71 were normolipemic (12 were type IIa and 14 were type IV). In each of the diabetic groups, low density and high density lipoproteins were enriched in triglyceride (p <0.01). These alterations in lipoprotein composition were not accounted for by differences in treatment modality, sex, race, age, or ponderal index. Thus the presence of diabetes itself appears to result in altered low and high density lipoprotein compositions. The mechanism(s) for the changes in the high density lipoproteins is(are) unknown, but in the light of present knowledge, the presence of triglyceride enriched lipoprotein could mean that a less dense moiety of low density lipoproteins may be accumulating in diabetic plasma, and this could result from altered rates of turnover of apolipoprotein B containing lipoproteins. Appropriate studies are needed to confirm this formulation.