Two cases of siblings with glycogen storage disease of the heart are reported. The clinical picture was characterized by progressive weakness, areflexia, and susceptibility to secondary infections. There was also progressive hepatomegaly and cardiomegaly. The pathologic findings correlated well with the clinical and were characterized by marked glycogen storage in skeletal, heart, and smooth muscle, the liver, and nerve cells of the central and autonomic nervous systems. In addition, many other tissues showed some glycogen storage. Storage in the skeletal muscle was so marked as to result in degeneration of the muscle fibers. The familial aspect of this disease is again noted. Suggestions regarding differential diagnosis are made.