Records were reviewed of 247 patients found to have isolated perimembranous ventricular septal defect (VSD) on cross-sectional echocardiography. Patients were separated into 2 groups: those in whom a perimembranous ventricular septal aneurysm (VSA) was associated with the VSD (group A, 77% of patients) and those in whom a VSA was not present (group B, 23%). The VSD was assessed by clinical (125 in group A, 24 in group B) and hemodynamic criteria (65 in group A, 33 in group B). The median follow-up period was 27 months (range 3 months to 25 years). In group A, the VSD closed spontaneously in 11% of the patients, improved clinically in 33% and required surgical closure in 11%. In group B, the VSD closed spontaneously in only 2%, improved clinically in 16% and required surgical closure in 47%. When considering larger VSDs only, 28% required surgery in group A, whereas 84% required surgery in group B (p less than 0.001). The VSA was found at the first echocardiographic examination in 94% of the studies. Thus, VSA is found in a large proportion of patients with perimembranous VSD. It occurs as an early phenomenon and is an important mechanism of closure. The presence of a VSA appears to confer a more favorable prognosis in perimembranous VSD except in patients with Down's syndrome.