An abnormal polysaccharide in the form of cytoplasmic spheroids was found in the nervous system and systemic organs of a man with a progressive neurological disorder characterized by onset at 47 years of age, severe weakness, sensory loss, and dementia. Results of biochemical analysis showed a marked increase in brain and heart polysaccharide that was resistant to digestion by a mixture of glucosidases and that exhibited an iodine-complex spectrum higher than that of normal glyocgen. Results of histochemical studies were consistent with the results of biochemical analysis and further defined the branching characteristics of the stored polysaccharide. Electron microscopy showed the cytoplasmic location of the spheroids, which were granular and filamentous.