The effectiveness of systemic deep hypothermia for myocardial protection was evaluated retrospectively in 36 consecutive children who underwent total correction of tetralogy of Fallot in the four-year period 1980 to 1984. Moderate hypothermia combined with potassium-induced cold cardioplegia and topical cardiac cooling was employed in 16 patients (Group A), and deep hypothermia together with cold cardioplegia and topical cooling was used in 20 patients (Group B). A higher incidence of spontaneous defibrillation, a higher postoperative right ventricular cardiac index, a significant decrease in the maximal requirement of isoproterenol hydrochloride, a significant increase in the mean urinary output, and much better operative results were obtained in Group B compared with Group A. Postmortem histopathological examination of the heart in 3 patients in Group A disclosed various degrees of hypoxic change in the myocardium, which were more pronounced in the right ventricle than in the left ventricle. It is concluded that the myocardial protection obtained with cold cardioplegia and topical cooling under moderate hypothermia may well be insufficient for repair of tetralogy of Fallot, a condition characterized by increased non-coronary blood flow to the myocardium and abundant collateral bronchial flow. However, when combined with systemic deep hypothermia, such myocardial protection is quite safe and effective.