To determine whether alterations in lipoprotein phospholipid composition might be an unrecognized factor that contributes to the unexplained acceleration of atherogenesis and the loss of sex-related protection from the development of coronary heart disease in women with insulin-dependent diabetes mellitus, we have estimated levels of neutral lipids, apolipoproteins (A-I, A-II, B), and free cholesterol (FC) in plasma and the four major phospholipid constituents of the very low-density lipoprotein + low-density lipoprotein and high-density lipoprotein (HDL) fractions in 12 ambulatory female patients with varying degrees of diabetic control. Although levels of triglyceride, cholesterol, HDL-cholesterol, and lipoprotein phospholipids in whole plasma of the patients with diabetes were similar to those in controls, their FC levels and FC/lecithin ratio, a recently described index of cardiovascular risk, both were abnormally increased (p less than 0.01). In the HDL-containing plasma fraction, concentrations of sphingomyelin, lecithin, and lysolecithin all were significantly reduced (p less than 0.05; p less than 0.01, and p less than 0.02, respectively). These compositional changes may be potentially atherogenic, because a reduction in the phospholipid content of HDL may impair its capacity to promote the efflux of cholesterol from cells, and the transfer of cholesterol ester from HDL to the larger apo-B-containing lipoproteins is inhibited when their content of FC is increased relative to phospholipid. These previously unrecognized qualitative defects, which are inapparent in the routine estimation of plasma lipids, may compromise reverse cholesterol transport and thereby promote atherogenesis in women with insulin-dependent diabetes mellitus.