In the past 7 years, 1967 through 1973, 12 patients who had interventricular septal defects from penetrating stab wounds of the thorax were treated at Detroit General Hospital. In 10, the defects were confirmed by cardiac catheterization. Four of the 12 patients were symptomatic and required surgery. Two of these patients had pulmonary-to-system flow ratios less than 2:1. Both these patients had left ventricular dysfunction as a result of coronary artery and myocardial damage from the penetrating trauma to the left ventricle. One of the 4 patients had an injury to the tricuspid valve; this is the first reported case of a traumatic interventricular septal defect associated with a left ventricle-right atrium shunt and tricuspid valve insufficiency. Another patient had a laceration of the left anterior descending coronary artery, in addition to the interventricular septal defect. Ligation of the injured artery resulted in a left ventricular aneurysm. Three patients had "multiple defects" in the muscular septum which proved to be a single interventricular septal defect in which the left to right shunt was divided into multiple jets by the trabeculae carneae of the right ventricle.