A unique diverticulum of the basal region of the right ventricle ruptured spontaneously, leading to unexpected death in an infant with persistent truncus arteriosus. Initial symptoms, including cyanosis and tachypnea, appeared to be corrected by digitalization. Later clinical manifestations resembling upper-respiratory tract infection also disappeared shortly before death. Roentgenographic examination showed enlargement of the heart, prominent pulmonary vascular markings, and, eventually, the diverticulum. Additional anatomic findings include agenesis of pulmonary artery, bronchial artery hypertrophy, and two congenital diverticula of right ventricle, with rupture and hemopericardium.