Cold potassium cardioplegia was used in 78 consecutive patients undergoing correction of complex congenital heart disease between 1977 and 1982. Ages ranged from 4 weeks to 21 years (mean 6.7 years). The anatomical diagnoses were: tetralogy of Fallot (33), common AV canal (12), pulmonary atresia with ventricular septal defect (VSD) with previous shunts (5), transposition of great arteries (5), total anomalous pulmonary venous return (3), complex VSD (4), and complex anomalies (16). There were 3 deaths (4%). Electron microscopy was performed on biopsy specimens taken from the hypertrophied or volume-loaded cardiac chamber (1) before perfusion, (2) after cardioplegia, and (3) 30 minutes after cardioplegia. It showed near normal myocardial ultrastructure after cardioplegic arrest, however there was minimal mitochondrial and intracellular edema after reperfusion. Post-operatively, 80% of the patients had spontaneous defibrillation and only 10% required brief inotropic support. The operative exposure was excellent. No patient developed heart block during repair.