During the 19 years from 1957 through 1975, there have been 106 patients under age 2 years who have undergone surgery for repair of a large ventricular septal defect at the University of Michigan Medical Center. The majority of the patients had either severe pulmonary hypertension or intractable congestive heart failure. Eighty-three infants survived operation; there has been one late death. The greatest mortality occurred in patients under age 6 months and in those with severe pulmonary hypertension. Surviving infants showed marked symptomatic improvement and change in growth patterns. Complications included the development of complete right bundle branch blodk or left anterior hemiblock in approximately 50 percent of patients and, in one instance, complete atrioventricular block. Forty-five patients have undergone cardiac catheterization 1 to 8 years postoperatively. Although 17 were found to have residual septal defects only 3 of these had a pulmonary to systemic flow ratio of 1.5:1 or more, and reoperation was accomplished without incident in these 3 patients and in 3 others with smaller shunts. With one exception, postoperative pulmonary arterial pressures and pulmonary to systemic vascular resistance ratios were normal or near normal, thus representing a significant contrast with findings in patients operated on after age 2 years. Whereas the complications of surgery appear no greater in the infant than in the older patient, many of the benefits can be realized only with operation at the earlier age.