Two cases in a family with Kufs' disease had lethal arrhythmias and heart muscle disease. Autopsy findings showed an abundant accumulation of lipofuscin-like lipopigments in most neurons in the central nervous system (CNS). The heart showed a slight increase in the accumulation of the lipofuscin-like lipopigments in the myocardial fibers, slight to severe fibrosis and infiltration of fat cells in the myocardium. The lipopigments both in the heart and in neurons of the CNS had curvilinear profiles on electron microscope and reacted immunohistochemically to polyclonal antibodies against subunit c of mitochondrial adenosine triphosphate (ATP) synthase. The degenerative process in this heart muscle disease might be attributable to the same metabolic abnormality as seen in the neuronal degeneration associated with Kufs' disease.