Mucolipidosis Type IV is a rare, autosomal recessive disorder characterized by corneal opacification, mental retardation, and delayed motor milestones. Whereas lysosomal storage material has been demonstrated in biopsied tissues and leukocytes, the complete autopsy pathology, including neuropathology, is unknown. The metabolic defect remains speculative. We report the general and neuropathologic findings of the only known autopsy. In the central nervous system, neuronal loss in the cerebral cortex, basal ganglia, deep cerebellar nuclei, and brainstem nuclei was marked by astrocytosis; the cytoplasm of residual neurons had brown granules. These granules were positive with periodic acid-Schiff, Concanavalia ensiformis, and Sudan black, but not with Luxol-fast blue. Ultrastructurally, neurons contained lysosomes laden with osmiophilic, amorphous and granular material, and few lamellated membrane structures. Hepatocytes, epithelia, endothelia, chondrocytes, and tissue macrophages also stained positively with Datura stramonium and Ricinus communis-I agglutinins, with renal glomeruli also staining with peanut agglutinin; most non-neural cells contained osmiophilic granules on toluidine blue-stained, plastic embedded sections, corresponding to lamellated membrane structures. These findings complement the previously reported ocular morphology and brain and liver biochemistry performed in the same patient, and suggest that the storage material in neurons differs from that in non-neural cells. Furthermore, the underlying defect is not likely to be a deficiency of a single enzyme (i.e. a lysosomal hydrolase).