We used intravascular ultrasound (IVUS) to compare the degree of coronary artery dilatation during the acute phase of Kawasaki disease with the extent of intima-medial thickening more than 10 years later. We wanted to determine if there was a threshold degree of dilatation that was highly predictive of later thickening. Twenty-eight patients with a mean age of 17.3 +/- 1.7 years were studied; the mean interval from the initial selective coronary angiography to the IVUS study was 15.0 +/- 1.6 years. We measured the maximum intima medial thickness of selected coronary arterial segments in IVUS images and measured the largest diameters of the corresponding coronary arterial segments in the initial coronary angiograms. A significant correlation was found between the initial diameters of the coronary arteries and the intima medial thickness more than 10 years later in the right coronary, the left anterior descending coronary, and the left circumflex arteries. The coefficient of correlation was 0.77 (n = 120, p < 0.0001), and for the bifurcation of the left coronary artery it was 0.50 (n = 26, p < 0.01). For this study, abnormal intima medial thickness was defined as more than 0.40 mm. When the initial coronary arterial dilatation exceeded 4.0 mm, the sensitivity was 28/31 (90%) and the specificity was 87/89 (98%) in the right coronary, the left anterior descending coronary, and the left circumflex arteries. For the bifurcation of the left coronary artery, the sensitivity was 14/21 (67%) and the specificity was 5/5 (100%).