The 38 adult patients admitted to the Natural History Study with a small ventricular defect had a generally favorable course, similar to that noted in children and adolescents. None died under medical or surgical management, and none deteriorated. Among patients treated medically, complete closure was demonstrated during the course of follow-up in one, and in the others, wo were catheterized, the shunt remained the same or decreased slightly. Significant vascular disease did not develop in any of the patients. Among seven surgically-treated patients for whom data were complete, the defect was completely closed in six. In 18 patients who had large ventricular septal defects and mild or moderate pulmonary vascular disease, no significant change occurred in the four who were managed medically. Of 14 who underwent surgery, two died; six of eight who were recatheterized had normal pulmonary arterial pressure, with no residual shunt in four and a trivial shunt in two. Two patients who had had moderate vascular disease preoperatively (RP/RS was 0.4 in both) had open defects and slightly higher resistance ratios at final catheterization. Of the 18 patients with Eisenmenger's syndrome, eight died during the study, including one of two who underwent surgery. The other surgical patient survived and showed significant hemodynamic improvement at final catheterization.