The purpose of this study was to compare the cardiopulmonary responses at rest and during exercise of children following surgical correction of tetralogy of Fallot. The physiologic measurements assessed at rest and during maximal treadmill exercise included diffusing capacity of the lung for carbon monoxide, heart rate, oxygen consumption, minute ventilation, and cardiac output. Twenty children with repaired tetralogy of Fallot served as the study group, and 20 normal boys and girls served as controls. Subjects with tetralogy of Fallot had significantly lower values than those of control subjects for maximal cardiac output and somewhat lower values for diffusing capacity of the lung for carbon monoxide and heart rate. We conclude that the exercise cardiac function of children with surgically corrected tetralogy of Fallot is somewhat diminished compared with that of control children. This information is necessary to establish safe, yet effective exercise guidelines for this select population.