Glycogen storage disease with normal acid maltase first reported by Danon et al. was characterized clinically by mental retardation, cardiomyopathy, and proximal myopathy. Since the first report, 17 patients have been reported including 5 patients from Japan. In this paper we described a 26-year-old man who had dilatated cardiomyopathy with a pacemaker implanted at age 22 years. He was admitted to our hospital complaining of easy fatigability in February 1992. Neurological findings showed that he had mental retardation. Serum CK, GOT, GPT and aldolase levels were elevated. Histopathological study of biopsied skeletal muscle showed intracytoplasmic vacuoles with increased acid phosphatase and slightly increased PAS positive material. Electron microscopic study revealed numerous glycogenosomes (autophagic vacuoles containing glycogen). These pathological findings were similar to acid maltase deficiency, but activities of carbohydrate metabolic enzyme including acid maltase activity were normal in the biopsied muscle. From these results, he was diagnosed as having glycogen storage disease with normal acid maltase. We also found abnormal platelet function and glycogen accumulation in the platelets, which have not been previously described. The disease is probably a systemic disorder affecting not only skeletal and cardiac muscles, but platelets.