An unusual case of diverticulum of the left ventricle is presented. The most distinctive feature was the presence of a circumferential mid-ventricular contraction ring with histologic abnormalities similar to those described in patients with asymmetric septal hypertrophy. This abnormal ring of tissue obstructed emptying of the ventricular apex and isolated it from the rest of the ventricle. The obstructed apex assumed the appearance of a muscular diverticulum, and the residual portion of the ventricle was markedly diminished in volume. In addition, the abnormal mid-ventricular hypertrophy interfered with papillary muscle function, producing mitral valve prolapse and mitral regurgitation. The patient's age was equally unusual; she was 65 at the time of diagnosis, probably the oldest known patient with ventricular diverticulum. She was treated surgically by transapical myotomy and myectomy of the mid-ventricular contraction ring, in order to increase effective ventricular volume. The relationship of this entity to other types of ventricular deiverticula and aneurysms, and to hypertrophic cardiomyopathy, is discussed.