We wanted to clarify the relationships between the degree of acute coronary artery dilation caused by Kawasaki disease and subsequent late calcification. Electron beam computed tomography (EBCT) was used to study 79 patients who had previously undergone selective coronary angiograms less than 100 days after the onset of Kawasaki disease. The EBCT was performed using an Imatron C-150 with a 100-ms exposure time and consecutive images at 6-mm intervals. The interval from the onset of Kawasaki disease to EBCT ranged from 2 to 242 months (median, 103 months). The maximum diameters of the right coronary, the left anterior descending, and the left circumflex arteries, as well as the bifurcation of the left coronary artery were measured in the initial coronary angiograms. A total of 250 branches, including 53 left coronary arteries, were measured, and the relationship between the degree of the initial coronary artery dilation and subsequent calcification in the branches and left coronary artery was analyzed. The coronary arterial diameter of all branches that eventually calcified was 6 mm or greater. The incidence of calcification in branches measuring 6 mm or greater on the initial coronary angiogram was 12% at 5 years, 44% at 10 years, and 94% at 20 years (n = 141). Dilation greater than 6 mm is associated with a high probability of late calcification.