To determine the timeframe in which coronary artery aneurysms (CAAs) caused by Kawasaki disease reach their maximum diameter, the timeframe in which they regress to normal size, and the cutoff point of the diameter for CAA regression. We reviewed 195 CAAs of the right coronary artery, left anterior descending artery, and left coronary artery measured by 2-dimensional echocardiography ≥5 times for 1 year after Kawasaki disease in 84 patients using medical records from 1995. The maximum diameters of CAAs were investigated retrospectively. The time to CAA regression using both absolute diameter and Z score were investigated. The cutoff points of the diameter of CAA regression in the 2 classifications were identified using receiver operator characteristic curve analysis. One year after Kawasaki disease, a CAA of <3.0 mm in absolute diameter and a Z score of <2.5 were defined as CAA regression. The time when CAAs reached their maximum diameter ranged from 11 days to 87 days, with a median of 35 days (n = 195). The time to CAA regression ranged from 41 to 386 days, with a median of 136 days in the absolute diameter classification (n = 92); 78% of CAA regression regressed by 200 days. The cutoff point for CAA regression at one year was 5.7 mm for the absolute diameter (area under the curve, 0.887; P < .0001; n = 190) and 9.5 for the Z score (area under the curve, 0.815; P < .0001; n = 195). CAAs with a smaller diameter regressed earlier, and most CAAs of <6 mm regressed by 6 months after Kawasaki disease.