The size of the coronary arteries parallels the ventricular mass, thus it may be abnormal in complete transposition beyond infancy and could be influenced by the arterial switch operation. To investigate this possibility, we measured the diameters of the right, left main trunk, anterior descending, and circumflex coronary arteries before and three to seven years (mean 4.8) after the arterial switch operation in 17 patients with a “normal” distribution of the coronary arteries (so-called Shaher type 1). The values were compared with 18 controls who had Kawasaki disease with no apparent coronary arterial disease. The right, left anterior descending, and circumflex arteries were smaller than control values before the operation. The post-/preoperative ratios of the diameter were 1.16±0.11 for the right coronary artery, 1.18±0.16 for the left main trunk, 1.20±0.18 for the left anterior descending artery, and 1.22±0.26 for the circumflex artery. There were no significant differences among these values. After surgery, the right coronary artery was larger, but the left coronary arteries were smaller in the patients than in the controls: 2.5±0.3 vs 2.0±0.2 mm for the right coronary artery; 2.4±0.3 vs 2.7±0.1 mm for the left main trunk; 1.9±0.2 vs 2.4±0.2 mm for the left anterior descending artery; 1.6±0.4 vs 2.2±0.5 mm for the circumflex artery, respectively. The posterior descending coronary artery originated from the right coronary artery in all patients. The total cross-sectional area of the right coronary, left anterior descending, and circumflex arteries was 9.7±2.4 mm2 in the patients, and 11.8±2.9 mm2 in the controls (p>O. 1), suggesting that the increased size of the right coronary artery compensates for the small left coronary arteries. We conclude that the arterial system in complete transposition, with a large right coronary artery and small left coronary system, remains smaller than normal even at midterm follow-up after anatomic repair despite normalization of left ventricular volume and muscle mass.