We report a 51-yr-old woman with late-onset progressive weakness affecting proximal limb muscles. Muscle biopsy revealed a vacuolar myopathy with accumulation of amylopectin-like polysaccharide resembling the polyglucosan found in type IV glycogenosis and adult-onset polyglucosan body disease. A biochemical study ruled out specific enzymatic defects known to cause storage of this abnormal material. Our case confirms the existence of a 'polyglucosan body myopathy' as a distinct clinicopathological entity in which the biochemical defect is unknown.