The aim of the present study was to evaluate the serum and lipoprotein levels of choline--containing phospholipids (lecithin, lysolecithin and sphingomyelin) in a group of type I diabetic patients in fair metabolic control. Forty-eight male adult diabetic patients without macroproteinuria were compared with 48 healthy controls matched for age and body weight. Total cholesterol and phospholipids were assessed in plasma and in apolipoprotein B (apoB) and non apoB containing lipoprotein particles (Lp B and Lp non B particles) separated using precipitation with Concanavalin A. When compared to the control group, diabetic patients have significantly lower serum phospholipids and phospholipids and cholesterol content of LpB particles. All the other parameters studied were similar in the two groups. The phospholipids/cholesterol ratio, calculated in LpB particles is also significantly reduced in the diabetic group. These results strongly suggest a depletion in the choline containing phospholipid content of LpB particles and therefore an alteration of the surface components of atherogenic particles in type I diabetes mellitus.