The development of 790 untreated patients affected by ventricular septal defect (VSD) has been the object of a 25-year study. Of these patients, 72% had had at least one catheterization; 13% had several. The mean observation interval is six years, and the average age at the latest data is 19.5 years. This study covers 4717 patient-years. For the entire population, the incidence rate or aortic regurgitation is 6.3% (4.3 for 1000 patient-years) and that of bacterial endocarditis is 3.7% (2.4 for 1000 patient-years). Twenty-five patients died, 15 of them between the ages of one and 39. Of the 499 cases with several clinical examinations, 71% remained stable. In 21%, changes suggesting some level of closure developed; in 7%, infundibular stenosis began to evolve and in 1% pulmonary vascular disease began to appear or became accentuated. These different rates are studied and discussed in relation to patients' age, VSD type, and various follow-up characteristics.