Lipid abnormalities in diabetic patients are likely to play an important role in the development of atherogenesis. These lipid disorders include not only quantitative but also qualitative abnormalities of lipoproteins which are potentially atherogenic. Both types are present in non-insulin-dependent diabetes (NIDDM) and poorly controlled insulin-dependent diabetes (IDDM), whereas only qualitative abnormalities are observed in well- and moderately well-controlled IDDM. The main quantitative abnormalities are increased triglyceride levels related to elevated VLDL and IDL and decreased HDL-cholesterol levels due to a drop in the HDL2 subfraction. The increase of triglyceride-rich lipoproteins in plasma is related to higher VLDL production by the liver and a decrease in their clearance. Metabolic abnormalities of triglyceride-rich lipoproteins are more pronounced in the postprandial period. The decrease in HDL-cholesterol is related to increased HDL catabolism. Qualitative abnormalities include changes in lipoprotein size (large VLDL, small LDL), increase of triglyceride content of LDL and HDL, glycation of apolipoproteins, and increased susceptibility of LDL to oxidation. These qualitative abnormalities impair the normal metabolism of lipoproteins and could thus promote atherogenesis. The physiopathology of lipid disorders in diabetes mellitus is multifactorial and still imperfectly known. However, such factors as hyperglycaemia and insulin resistance (in NIDDM) are likely to play an important role.