A 7 1/2-year-old girl had exercise intolerance and exertional dyspnea. Four months later, congestive heart failure developed, with recurrent chylous pleural effusions, and she died at age 8 1/2 years. Endomyocardial biopsy tissue showed abundant PAS-positive, diastase-resistant cytoplasmic deposits. Similar inclusions were seen in muscle, skin, and liver specimens. Postmortem studies showed that the abnormal polysaccharide was especially abundant in heart and muscle, but was also present in all other tissues, including the central nervous system. Glycogen isolated from heart, muscle, and spinal cord showed a shift of the iodine spectrum toward higher than normal wavelengths. Branching enzyme activity was lacking in the muscle biopsy specimen and in all postmortem tissues; glycogenolytic enzymes had normal activities. These studies show that cardiomyopathy can be the first symptom of generalized branching enzyme deficiency and that the degree of accumulation of the abnormal polysaccharide may vary in different tissues.