Histological material obtained at autopsy from 35 patients over three months of age with complete transposition of the great arteries (TGA) was examined. Two of six patients less than a year of age with an intact ventricular septum and closed ductus arteriosus were found to have pulmonary vascular changes of at least grade 3 severity; in addition, two of seven patients with a large ventricular septal defect in this age group showed changes of similar severity. The reported low incidence of marked pulmonary vascular changes in patients with only an interatrial communication dying during the first year of life would appear to be due in part to the high proportion of cases less than three months old in whom there was not sufficient time for such changes to develop. Although the incidence is not as high as that found beyond a year of age, it is sufficiently high to influence the management of infants beyond three months of age. Hemodynamic studies were performed following atrial baffle repair in 34 patients. All but one of the 16 patients operated upon prior to two years of age had a pulmonary arteriolar resistance (Rpa) less than 3 units M2 at the time of review, including three with a large communication at ventricular or great vessel level who underwent surgery before two months of age. Repair at an older age did not preclude a low Rpa on follow-up, but in 11 of the 18 patients who were two or more years of age at the time of repair it was greater than 3 units M2. None of these latter patients had a large ventricular septal defect or patent ductus arteriosus. Comparison of postoperative hemodynamic data with autopsy studies would suggest that advanced pulmonary vascular disease is associated with earlier death among patients with complete transposition of the great arteries.