A 13-year-old boy with mental retardation developed idiopathic cardiomyopathy and glycogen storage myopathy, but with normal lysosomal enzyme activities, consistent with a syndrome of lysosomal glycogen storage disease with normal acid maltase coined by Danon et al (1981). He was in good health except for WPW syndrome diagnosed at 7 years of age. He had heart murmur with abnormal ECG, elevated serum GOT, GPT, LDH, CK and aldolase levels. An echocardiogram showed obstructive hypertrophic cardiomyopathy. Lysosomal enzyme activities including acid alpha-glucosidase in fibroblasts were within normal limits. In the biopsied biceps brachii muscle, there was a mild variation in fiber size. An approximately 10 percent of myofibers had tiny vacuoles which contained periodic acid Schiff positive granules and were slightly high in acid phosphatase activity. The vacuoles were encircled by membranes with high neuron specific enolase (NSE) and acethylcholin-esterase (AchE) activities. On electron microscopy, numerous autophagic vacuoles scavenging glycogen granules were recognized as seen in acid maltase deficiency. Because the vacuolar membranes were high in NSE and AchE activities, lysosomal membrane formation from the cell membrane may be defective. When one has a patient with mild to moderate mental retardation, idiopathic hypertrophic cardiomyopathy and high serum CK level, muscle biopsy must be performed to rule out the present disorder.