The light- and electron-microscopical findings in the skeletal muscle and conjunctiva of 6 patients with Marinesco-Sjögren syndrome (MSS) were presented. All patients were related and showed the cardinal clinical symptoms of the syndrome: congenital cataracts, mental retardation, delayed statomotor development, and cerebellar ataxia. The most prominent alteration found in the skeletal muscle of four patients was extensive neurogenic atrophy with conspicuous groups of atrophic muscle fibers. Additional findings were vacuolar degeneration and secondary, unspecific changes like slight mitochondrial alterations and increased variability in muscle fiber calibers. The ultrastructural examination revealed double-membrane structures near to, but not in direct contact with the nucleus. The nuclear changes described earlier (7, 20) were not found in any of the cases. Conjunctival biopsies revealed a marked increase in the number of lysosomes in fibroblasts. In summary, to the well known myopathic damage of muscles in MSS a neurogenic component should be added. Disturbed lysosomal function is to be considered as the basic abnormality, though the enzyme defect has not yet been identified. In cases of clinically suspected MSS, examination of a conjunctival biopsy is highly recommended.