Postmortem studies have shown a relationship between diabetic state and atherosclerotic arterial lesions in adolescents. The aim of the present study was to determine the presence of increased subclinical atherosclerosis (measured as carotid intima-media thickness [IMT]) and its risk factors, including lipoprotein oxidation, in children with type 1 diabetes. We measured carotid IMT using high-resolution ultrasound in 85 children (mean age, 11 +/- 2 years): 50 with type 1 diabetes (mean duration, 4.4 +/- 3.0 years) and 35 healthy control subjects matched for age, sex, and body size. The susceptibility of LDL to oxidation was determined by measuring the formation of conjugated dienes induced by Cu(2+) in 42 children (21 with diabetes and 21 control subjects). The mean carotid IMT was increased in children with diabetes (0.47 +/- 0.04 vs. 0.42 +/- 0.04 mm; P < 0.0001). Total cholesterol and LDL cholesterol concentrations were similar between the groups, but the children with diabetes had increased LDL diene formation rate (0.49 +/- 0.06 vs. 0.45 +/- 0.07 micromol/min; P < 0.05), suggesting increased in vitro LDL oxidizability. In a multivariate model for all subjects, the independent correlates for IMT were the diabetic state (P < 0.001), LDL cholesterol level (P < 0.001), and systolic blood pressure (P < 0.001). In children with diabetes but not in control subjects, LDL oxidizability correlated significantly with mean IMT (r = 0.47, P < 0.05), and this relationship remained significant after controlling for LDL cholesterol level. We conclude that type 1 diabetes is an independent risk factor for increased carotid IMT in children. These data also suggest that increased oxidative modification of LDL may be related to early structural atherosclerotic vascular changes in children with diabetes.