We prospectively evaluated 49 consecutive hospital survivors of the arterial switch operation for complete transposition and intact ventricular septum by clinical examination, echocardiography, cardiac catheterization, 12-lead and 24-h Holter ECG. The mean length of follow-up was 40 +/- 18 months. Forty-six children are clinically asymptomatic without medication, 2 died due to coronary related left ventricular dysfunction 3 and 12 months after surgery, and 1 required reoperation because of severe bilateral pulmonary branch stenoses. Except for this case, cardiac catheterization (n = 23) revealed a mean gradient of only 17 +/- 8 mmHg between the right ventricle and distal pulmonary arteries. Left ventricular end-diastolic volume was within normal limits except for 2 cases with volumes slightly below normal, the mean ejection fraction was 78 +/- 5%, and end-diastolic and end-systolic ventricular shapes were normal. The mean cardiac index was 4.14 +/- 0.69 l/min/m2. Left ventricular end-systolic wall stress to velocity of fiber shortening relation was normal in all cases examined (n = 15), indicating normal myocardial contractility. Significant neoaortic valve insufficiency was never observed despite considerably enlarged aortic roots. Twenty-four-hour Holter ECG records (n = 46) provided no evidence of serious atrial arrhythmias, especially sinus node dysfunction. These encouraging intermediate-term results make the arterial switch operation the treatment of choice at present, for neonates with simple transposition.