Histological and ultrastructural findings observed throughout the nervous system and the extranervous organs in a case of sialidosis type I, also known as normosomatic group, are reported. The patient was a 22-year-old male with non-familial progressive myoclonus, macular cherry-red spot, moderate cerebellar syndrome and normal intelligence. Biochemical study showed an alpha-N-acetylneuraminidase deficiency in cultured fibroblasts. A complete and early autopsy was performed. Neuropathological study showed two prominent lesions: the first one was a fine cytoplasmatic vacuolation in several neurons of the cortex, basal ganglia and thalamus and the second one was a diffuse neuronal intracytoplasmic storage of lipofuscin-like pigment (LLP). As for the extranervous organs the main light and electron microscope findings were observed in the hepatocytes and in the Kupffer's cells, which showed an enlarged cytoplasm and lipopigment granules in different amount. Vacuoles containing dense lamellar bodies were found in tubular epithelial cells of the kidney. To our knowledge this is the first complete autoptic study of a case of sialidosis type I.