Diabetes is associated with increased morbidity and mortality resulting from cardiovascular disease in general and with increased risk of coronary heart disease in particular (1). Diabetes also produces disturbances of lipid profiles, especially an increased susceptibility to lipid peroxidation (2), which might be related to the increased incidence of atherosclerosis in diabetes (3). The oxidation of LDL appears to be involved in the development of atherosclerosis (4). The oxidation of LDL leads to alteration of the LDL apolipoprotein B (apoB)  recognition site and in the unregulated uptake of LDL by macrophages via the scavenger receptor. The subsequent accumulation of cholesterol-loaded macrophages (foam cells) in the subendothelium leads to the formation of fatty streaks and atherosclerotic plaques (5). High blood glucose is commonly associated with increased oxidative changes in LDL (6). Hyperlipidemia, which is often present in diabetes mellitus, is also associated with oxidation of LDL. However, in contrast to the general population, cardiovascular disease is equally common in both men and women suffering from diabetes mellitus, and no classical risk factor of cardiovascular disease had been linked to this observation. A previous study demonstrated that the LDL of patients with poorly controlled type 1 diabetes mellitus was unusually sensitive to oxidation (7). However, it has also recently been demonstrated that women with type 1 diabetes mellitus have an increased degree of lipid peroxidation that is independent of their blood glucose control (8), although there is no information available about the situation in type 2 diabetic women. The greater cardiovascular risk in diabetic women could be related to an increased susceptibility of LDL to oxidation. We determined the influence of diabetes mellitus, hyperlipidemia, and gender on the susceptibility of LDL to oxidation in patients with type 1 or type 2 diabetes mellitus. We assessed the ability of LDL to generate peroxides by measuring the formation of thiobarituric acid-reactive substances (TBARS) in vitro in groups of men and women with type 1 or type 2 diabetes mellitus and compared them to control groups.