Background and Purpose: Ethnic differences in cardiovascular disease (CVD) morbidity and mortality have been observed in US adults. However, little data exist on differences in indices of preclinical atherosclerosis such as carotid wall intima media thickness (IMT) for US non-Hispanic whites, Hispanics, and blacks. This study was undertaken to determine whether there were ethnic differences in carotid wall IMT. Methods: Internal carotid artery (ICA) IMT and common carotid artery (CCA) IMT, indices of atherosclerosis, were assessed with the use of B mode ultrasound in 1020 nondiabetic participants in the Insulin Resistance Atherosclerosis Study, a multicenter study designed to examine the association between insulin resistance and carotid atherosclerosis. The study included 281 blacks, 329 Hispanics, and 410 non-Hispanic whites aged 40 to 69 years. Results: Blacks had significantly greater CCA IMT than non-Hispanic whites (865 versus 808 μm); this remained significant after adjustment for major CVD risk factors and insulin sensitivity (864 versus 823 μm). There were no significant differences in ICA IMT between blacks and non Hispanic whites. Hispanics had significantly lesser CCA IMT than non-Hispanic whites (749 versus 776 μm), and these differences remained significant after adjustment for traditional cardiovascular risk factors and insulin sensitivity (750 versus 778 μm). There were no significant differences in ICA IMT between non-Hispanic whites and Hispanics. Conclusions: We conclude that ethnic differences exist in CCA but not in ICA IMT in nondiabetic subjects. These differences in IMT, which are indicators of atherosclerosis, are a noninvasive measure that is consistent with some of the data on clinical end points. These differences may be associated with the observed differences in CVD morbidity and mortality among major ethnic groups in the United States.