Seven cases of anomalous muscle bundles of the right ventricle are presented. In one patient having an associated ventricular septal defect, spaces between the anomalous muscle were confused surgically with the ventricular septal defect. In the remaining 6 cases, the anomalous muscle bundle caused obstruction within the right ventricular cavity. Five of these patients had associated ventricular septal defect, with a clinical picture resembling that of the cyanotic or acyanotic tetralogy of Fallot. The cases demonstrate that the surgeon who exposes the right ventricle to relieve an obstructive lesion in the right ventricular infundibulum or in the pulmonary valve, may find no such lesions in these locations but instead may be dealing with an obstruction more proximal than these areas. Relief of the obstruction within the right ventricle may be attained by resecting the anomalous muscle mass. In one patient with intact ventricular septum and pulmonary valvular stenosis, an anomalous muscle bundle within the right ventricle which lay proximal to the infundibulum was responsible for persistent right ventricular obstruction after pulmonary valvotomy. Anomalous muscle bundles in the right ventricle may be suspected when, in the angiocardiogram, they cause filling defects within the right ventricle.