The cardiovascular abnormalities were investigated in two unrelated hemizygous males with Fabry disease who had clinical mitral insufficiency. Postmortem examination of their hearts revealed anatomic, ultrastructural and biochemical abnormalities resulting from defective activity of the lysosomal enzyme, α galactosidase A. The ultrastructural and biochemical studies demonstrated the marked accumulation of the major glycosphingolipid substrate, trihexosyl ceramide, in the lysosomes of all the cardiac tissues examined; the greatest concentrations were found in the mitral valve and left ventricular myocardium. Intriguingly, digalactosyl ceramide, a glycosphingolipid substrate not detectable in normal lung, vessel or cardiac tissues, was found increased only in the lung and right heart tissues. Morphologic and chemical examination of cardiac and systemic vessels demonstrated accumulation of trihexosyl ceramide in lysosomes of the vascular endothelium. These studies demonstrate that the progressive accumulation of trihexosyl ceramide in the lysosomes of the cardiac structures and vascular system leads to the multiple cardiovascular manifestations of Fabry disease.