A 4-year-old girl had the insidious onset of congestive heart failure without apparent cause. Evaluation by echocardiography, thallous chloride TI 201 scintigraphy, and angiography suggested the presence of either a neoplasm or restrictive cardiomyopathy with a localized mass effect on the left ventricle. Pathological specimens obtained by transvascular endomyocardial biopsy and at surgery defined the pathology to be restrictive cardiomyopathy. Analysis of myocardium by electron microscopy demonstrated a previously undescribed abnormality of the contractile elements involving the myofilaments and Z bands, with generalized secondary glycogen deposition.