Five cats with feline α-L-iduronidase-deficient mucopolysaccharidosis were studied. Membrane-bound cytoplasmic inclusions were present in central nervous system neurons, hepatocytes, chondrocytes, vascular and splenic smooth muscle cells, bone marrow leukocytes, and fibroblasts of the skin, eye, and cardiac valves. The lesions in these cats closely resemble those described in human patients with mucopolysaccharidosis I H (Hurler syndrome).