The light and electron microscopic findings in the heart of a patient with Fabry's disease are described. The study revealed that all cardiac tissues, including the conducting tissues and the valves, were involved. The latter finding was of particular interest since the patient was known to have mitral insufficiency. The findings of diffuse ballooning of the mitral valve with localized "overshoot" and massive glycolipid storage in the valve substance suggest that the abnormal storage process was itself responsible for the valvular insufficiency. The widespread involvement of the myocardium and conducting tissues is consistent with the elelctrocardiographic changes indicating infarction, although myocardial necrosis was not observed. The findings in this case suggest that the cardiac manifestations in Fabry's disease can be either primary, that is, directly related to the enzyme deficiency, or secondary, that is, evolving with time as a consequence of the disease.