Fifty infants ranging in age from 13 days to 18 months (mean age 6 months) and weighing from 1.7 to 8.2 kg (mean weight 4.5 kg) underwent patch closure of a ventricular septal defect (VSD) with use of deep hypothermic circulatory arrest. Seventeen infants were under 3 months of age. The principal indication for operation was intractable chronic congestive heart failure; All infants were below the third percentile for weight preoperatively; Three patients (6%) died postoperatively within the second month of life. There was no late mortality. Seven infants (14%) had seizures; these were associated with a low output state in 2 infants, with hypoxic episodes in 4 infants, and occurred postoperatively in 1 infant. Postoperatively, 8 (17%) of the surviving infants developed right bundle-branch block and left anterior hemiblock, and 16 (32%) developed right bundle-branch block alone. One year postoperatively, catheterization studies in 24 children revealed normal pulmonary artery pressure and pulmonary vascular resistance in all; there were no significant residual ventricular septal defects. Because of these results we continue to be enthusiastic about primary closure of VSD irrespective of age or weightk0