That abnormalities of lipoprotein metabolism occur frequently in patients with diabetes is well recognized. However, the effect of diabetes on lipoprotein metabolism is extremely complicated. For example, insulin-dependent diabetes mellitus (IDDM) and non-insulin-dependent diabetes mellitus (NIDDM) are quite different syndromes in many respects, and this is also true of their impact on lipoprotein metabolism. Furthermore, the changes in lipoprotein metabolism that can be seen even within each of these broad general syndromes are not uniform. Consequently, it is difficult to define a central theme that permits an organized discussion of the lipoprotein abnormalities present in diabetes. This is certainly the case in regard to the relationship between lipoprotein metabolism and magnitude of hyperglycemia, and great differences in lipoprotein metabolism between patients can be seen when they have similar plasma glucose concentration. One possible solution to this dilemma is to focus on the relationship between insulin and lipoprotein metabolism in patients with diabetes, and this is the approach we have selected. This decision is based upon the view that the characteristic changes in lipoprotein metabolism that have been described in association with the various diabetic syndromes are best understood when viewed in context of the changes in insulin action and circulating insulin level seen in these clinical situations. Implicit in this formulation is the belief that changes in insulin action and concentration play a central role in the regulation