Atherosclerosis is considered to be the major complication of diabetes mellitus. Since diabetic patients have increased blood levels of lipid peroxidation products we investigated whether the susceptibility of blood components to oxidation is altered in this disease. We analysed the parameters characterizing the extent of oxidative change and the antioxidant status of low density lipoprotein (LDL) and high density lipoprotein (HDL) in a group of diabetic patients and in a control population. LDL oxidizability was significantly higher for patients (P = 0.001) than for individuals in the control group. There were no significant differences in the α-tocopherol content or levels of preformed peroxides in LDL samples isolated from diabetic patients and control individuals which could account for this effect. Similarly, LDL glycation, common in diabetes mellitus, was not responsible, since LDL glycated in vitro was more rather than less resistant to oxidation. Even the presence of unbound glucose at normal or elevated physiological concentrations had a delaying effect on the oxidation of LDL. The increased oxidizability of LDL isolated from diabetic patients could be reduced to control levels by a 6-week standard treatment with Probucol™, originally applied to reduce their blood cholesterol.