Clinical data on 12 patients under 6 months of age are presented together with the necropsy evidence of truncus arteriosus. Five patients had normal truncal valves and seven had markedly deformed and incompetent valves. Among the group of infants with truncal valve insufficiency, the cause of death was refractory congestive heart failure in each instance, whereas the patients with normal truncal valves succumbed from other causes.Pathologic examination of the abnormal truncal valves revealed cusps which were thickened by fibrous nodules. Combined heart-lung weight in patients with deformed truncal valves had a mean increase of 85% above normal values, while in the group of patients with normal truncal valves the mean increase was 17%. Similar results were observed when liver weights were compared.The major clinical findings among the group of patients with truncal insufficiency were bounding peripheral arterial pulses, early diastolic decrescendo murmurs, and congestive heart failure which was refractory to medical treatment and, in one instance, surgical palliation. It is suggested from the data that deformities of the truncal valves are a primary cause of early mortality among infants with truncus arteriosus.