Mitral valve, coronary arteries, cartilage, and liver were studied by light and electron microscopy in a 15 year old boy with Morquio's syndrome, a genetic mucopolysaccharidosis, in which a deficiency of lysosomal hexosamine sulfatase is associated with accumulations of keratan sulfate in various organs. Coronary artery intimal sclerosis was a prominent feature of this disorder. Ultrastructural examination revealed numerous intimal smooth muscle cells containing storage vacuoles consistent with lysosomes. This was associated with marked interstitial deposition of collagen, elastin, and basement membrane material. Recent studies of human and experimental atherosclerosis have demonstrated the accumulation of cholesterol within vascular smooth muscle cell lysosomes. Intralysosomal accumulation of substrates other than cholesterol is also associated with vascular intimal sclerosis in genetic lysosomal disorders such as Fabry's disease and Hurler's syndrome. Lysosomal storage of undegraded substrate may be an important pathogenetic mechanism in the development of sclerotic vascular lesions.