Between April 1970 and December 1977, 134 infants aged 2 days to 12 months underwent open heart surgery using profound hypothermia and total circulatory arrest. The technique of bypass (core) cooling is described. Results are reviewed for 4 principal diagnoses: 'simple' transposition of the great vessels, total anomalous pulmonary venous connection, ventricular septal defect, and Fallot's tetralogy. A mortality of 44 per cent in 32 cases during the first 3-year period has been reduced to 22 per cent in 102 cases during the subsequent 5 years. The overall mortality for the entire period was 28 per cent. The policy for the management of each diagnostic group is outlined.