The light- and electron-microscopical findings in the skeletal muscle of a patient with Marinesco-Sjogren syndrome (MSS) are presented. Muscle biopsy specimens showed myopathy with slightly dystrophic changes including variation in fiber size, muscle fiber necrosis, regeneration and rimmed vacuole formation. Fiber type analysis with myosin ATPase staining showed a mild increase in type 2C fibers. Electron microscopy revealed autophagic vacuoles with numerous myeloid bodies, and a unique dense membranous structure associated with nuclei. We consider that this unique membranous structure is an important feature in the muscle pathology of MSS.