• The natural course of patients with ventricular septal defects is not known. During the past 10 years, 27 autopsies were performed on the authors' 336 patients with this defect. Twenty-six of these patients were under 6 months of age. It has been observed repeatedly that some infants with large ventricular septal defects manifest marked improvement in their symptomatology after their first year of life. In an attempt to explain these observations, clinical follow-up examinations were combined with cardiac recatheterization and angiocardiography, especially on those patients who had their initial studies during their first year of life. So far, 10 patients have been restudied and recatheterized. The time interval between their first cardiac catheterization and the recent ones varied from three to five years. Two of these patients now present typical clinical and hemodynamic data of ventricular septal defect with infundibular stenosis with left-to-right shunts. The third patient shows typical findings of tetralogy of with marked cyanosis and predominant right-to-left shunt. The frequency with which this natural transformation of ventricular septal defects into apparently different entities occurs is at present not known. The concept of the unity of origin of the three entities has been demonstrated in these patients, and this concept should lead to a re-evaluation of our knowledge concerning the diagnosis and medical and surgical treatment of congenital cardiac defects.