Abstract Sixty-two infants with ventricular septal defects were first catheterized under 1 year of age and followed up for one to five years; 40 were recatheterized. Thirty-six infants were born in the Bronx Municipal Hospital Center or Lincoln Hospital (local group). These patients were unselected, in that the decision to catheterize them was based only on the clinical diagnosis of a ventricular septal defect and not on an assessment of its size or the presence of symptoms. As far as we know, these infants include most if not all of the children born with ventricular septal defects in these hospitals during the period of study. With this assumption, the incidence of ventricular septal defects per 1,000 live births was 0.94 for full term infants, 4.51 for premature infants, and 1.35 for the whole group. Twenty-six other infants were referred from other hospitals or doctors (referred group), and, compared to the local group, they had larger defects, were more often in congestive heart failure and were more often catheterized after 6 months of age. In the whole series, 31 of 62 had congestive heart failure. This began before 6 months of age in all and occurred much earlier in premature than in full term infants. In the local group, spontaneous functional closure of the ventricular septal defect took place in 13 of 36 (36%); this was proved by recatheterization in 10 and autopsy in 1. Marked decrease in size of the defect took place in another 10 (28%). In the referred group (26 cases) there were 2 (8%) spontaneous closures and 7 (27%) reductions in defect size. For the whole series, 52 per cent of the patients had defects which were known to have closed or become smaller; 32 per cent were doing well clinically (most were not recatheterized so that some of these defects might have become smaller); and only 16 per cent of the infants were seriously affected by their lesions. Complete closure occurred between 7 and 12 months of age, most often when the defect was small, but it could occur with large defects. Decrease in defect size, however, occurred as frequently with large as with small defects, and was found in many children with congestive heart failure and pulmonary arterial hypertension. The 10 infants with large ventricular septal defects who did not do well included 4 who had pulmonary arterial banding for severe, uncontrolled congestive heart failure (2 died); 1 premature baby who died at 6 weeks of age with severe congestive heart failure; and 5 with a very high pulmonary vascular resistance. One of these 5 had a high pulmonary vascular resistance when 1 month old, and this did not change significantly at 5 and 13 months of age. The other 4, however, had low pulmonary vascular resistance when first catheterized at 3, 6, 6 and 11 months of age, respectively, and significant rises of resistance to pathologic levels when recatheterized at 9, 15, 26 and 40 months of age, respectively. This suggests that a rise in pulmonary vascular resistance in infancy is not rare when the ventricular septal defect is big and that, even in these children, the pulmonary vascular resistance does not usually persist at the high level present at birth but first falls post natally to normal levels before rising. These 5 infants all had large left to right shunts, high left atrial pressures and marked pulmonary arterial hypertension. In 3 out of the 4 whose pulmonary vascular resistances rose after the first catheterization, there were no distinctive clinical indications of this rise which was detected only at recatheterization. The other child had increasing right ventricular hypertrophy on the electrocardiogram. From these findings we suggest that, since in infancy spontaneous closure or reduction in size of the ventricular septal defect is so common, most infants should be treated conservatively in the hope that spontaneous improvement will occur. If the defect is very big and the child is in severe congestive heart failure which cannot be well controlled, then surgical closure of the defect or banding of the pulmonary artery should be done. If, however, the defect is big but congestive heart failure can be controlled medically, then the infant should be followed up with the hope that spontaneous improvement will occur. Whether clinical improvement occurs or not, the infant should be recatheterized six to nine months after the initial catheterization to determine if a rise in pulmonary vascular resistance has occurred; if it has then surgery should be advised.