Thallium-201 myocardial imaging was performed for 77 patients with congenital heart disease to evaluate myocardial damage following open-heart surgery. There were 23 patients with ventricular septal defects, 33 with tetralogy of Fallot, 12 with atrial septal defects, and nine with other diseases. The imaging results were compared with the angiographic and electrocardiographic findings. Patchy perfusion defects or images of poor thallium uptake were noted in five of 77 patients, among whom four had accompanying serious ventricular arrhythmias. This suggested a close correlation of the post-operative ventricular arrhythmias with myocardial damage. For 20 of 77 patients, vents were inserted through the left ventricle, and for the remaining 57 through the left atrium. Perfusion defects were found at the apex in three, and left ventriculography showed a paradoxical motion of the apex in five in patients whose vents were inserted through the left ventricle. Sensitivity and specificity in detecting myocardial damage of the apex were 60% and 98%, respectively. In one case, myocardial infarction was demonstrated immediately after the repair of the ventricular septal defect. Myocardial imaging was useful in evaluating the infarcted area and for follow-up study. Thus, thallium-201 myocardial imaging was regarded useful for assessing myocardial damage following open-heart surgery in congenital heart disease patients.