BACKGROUND: There have been recent reports that the fat distribution within skeletal muscle and the amount of muscle mass are associated with insulin resistance and the development of type 2 diabetes mellitus (T2DM). This study evaluated the impacts of visceral fat and thigh muscle from patients with T2DM and healthy subjects on atherosclerosis and insulin resistance. METHODS: Forty-two patients with newly-developed T2DM and 11 healthy subjects were selected for the study. The diabetic patients were subdivided into two groups, those under 40 years of age, as the young T2DM (n=21) group, and 40 years-old or greater, as the old T2DM (n=21) group. CT scans were obtained for all patients at the L4-L5 level and at the mid-portion between the greater trochanter and upper margin patella. The carotid intima-media thickness (IMT) was also measured using high resolution B-mode ultrasonography. RESULTS: The mean visceral fat area (VFA) in the old T2DM group was 169.4+/-13.2cm2, which was significantly greater than that found in the healthy subjects (67.9+/-7.92cm2, P<0.001) and young T2DM group (127.1+/-10.4cm2, P<0.05). The mean visceral fat to normal density muscle area ratio (VMNR) in the old T2DM group was 1.50+/-0.19, which was greater than in the healthy subjects (0.46+/-0.52, P<0.001) and young T2DM group (1.01+/-0.10, P<0.05). The total thigh muscle areas in the young and old T2DM groups were smaller than that in the healthy subjects, but without statistical significance. VMNR showed a positive correlation with the IMT and HOMA-IR. However, the total thigh muscle area was negatively correlated with the IMT. The normal density muscle area also showed significant negative correlations with the IMT and HOMA-IR. In a multiple regression analysis, age and VMNR were the most important independent risk factors of an increased carotid IMT. CONCLUSION: This study showed that the role of thigh muscle, as well as that of visceral fat, played a very important role in the occurrence of atherosclerosis. VMNR was found to be an especially important independent factor for an increased carotid IMT.