Purpose: To characterize the intercorrelation of changes in intimal-medial thickness (IMT) among carotid artery sites and examine the influence of diabetes and sex on these correlations. Methods: Ultrasonographic IMT measurements from the near and far walls of common and internal segments in the arteries of 1207 participants of the Insulin Resistance Atherosclerosis Study (IRAS) were analyzed to estimate the underlying correlations of IMT changes (average of 5.2 years) among artery sites after correction for measurement error. Results: Differences in IMT progression associated with diabetes and sex were evident at all sites. IMT changes were strongly correlated between left and right arteries (r=0.32-0.73) and near and far walls (r=0.42-0.87). Correlations of IMT changes at corresponding sites of internal versus common segments were reduced from r=0.32 among normal individuals to r=0.09 among those with diabetes, but were similar for women and men. Conclusions: IMT progression is a diffuse process involving all carotid artery sites. Diabetes does not influence the bilateral and radial nature of progression, but reduces the correlation between distal sites within an artery. Pooling measurements across arteries and walls often yields the most efficient analyses. © 2003 Elsevier Ireland Ltd. All rights reserved.