Lysosomal glycogen storage in muscle with normal acid maltase activity is a rare inherited condition characterized by cardiomyopathy, mental retardation and mild myopathy in males, but generally only cardiomyopathy in females. Three cases (index case, his sister and her son) are described in a family with at least two other affected members. The index case underwent a successful heart transplant. The sister has cardiac involvement, myopathic changes and mental impairment--to our knowledge the first report of multisystem involvement in a female. We propose that skeletal muscle should be examined in young patients with hypertrophic cardiomyopathy. Furthermore, female relatives of males with the disease should be investigated for cardiomyopathy; they would be excellent candidates for life-saving heart transplant, since myopathy and mental retardation, if clinically evident, are mild.