The aim of this study was to assess the evolution of ventricular septal defects (VSD) with respect to their initial echocardiographic appearances: size and site. One hundred consecutive neonates under 3 months of age with a typical VSD murmur, were included in a prospective study. Malformation syndromes and associated cardiac disease were criteria of exclusion. Disappearance of the murmur was indicative of closure of the VSD. The VSDs were classified as: small (diameter less than or equal to 3 mm), medium (3 to 6 mm) and large (greater than 6 mm). Twelve children were lost to follow-up; the remainder were followed up for an average of 35 months. The VSDs were muscular (39%), membranous (37%), infundibular (2%) or unlocalised (22%). Of the muscular VSDs, 85% were small and 15% were medium-sized; 48% of membranous VSDs were small, 43% were medium and 9% large-sized. Spontaneous closure was observed in 64% of cases but this figure varied with the size and site of the VSD. The prognosis of unlocalised VSDs was excellent (100% spontaneous closures at 1 year), as was that of small muscular VSDs (89% spontaneous closures at 1 year). The prognosis of medium size muscular and small membranous VSDs was less favourable: 40% closures at 1 year, 50% closures at 2 years respectively. The closure of medium-sized membranous VSDs was slower: 7% at 2 years. It was essentially in this group and that of large-sized membranous VSDs that the surgical indications were mostly to be found. No spontaneous closures were observed in the large membranous or the infundibular VSDs.