Of approximately 22,000 live births in the region under study during the last 4 years, ventricular septal defect (VSD) was identified as the primary or isolated congenital heart lesion in 124 infants who were followed up from birth for a minimum of 1 year (incidence, 5.7 per 1,000 live births). Doppler color flow mapping was performed in 93 of 124 patients; 47 had a muscular VSD and 46 had a perimembranous VSD. Only 1 patient had 2 muscular VSDs. None had a subpulmonic type of defect. Of 124 patients, 14 were lost to follow-up. Spontaneous closure was seen in 18 patients (42%) in the muscular group, in 9 (23%) in the perimembranous group and in 10 patients (37%) in the unclassified group by the end of the first year. The overall rate of spontaneous closure was 34% by the end of the first year. Congestive heart failure developed in 2 of 46 patients with muscular VSD and in 12 of 47 patients with perimembranous VSD. In the first year, 2 patients with muscular VSD as opposed to 5 with perimembranous VSD required surgery. Doppler color flow mapping is a valuable aid in the diagnosis of VSD and may be one reason for the observed increase in the incidence of VSD. The overall prognosis appeared much better in the muscular than the perimembranous type of VSD.