We present a clinical study and muscle biopsy of a 13-year-old female who suffered hypertrophic cardiomyopathy, hepatosplenomegaly and myopathy of prolonged evolution. The muscle biopsy showed a glycogenosis with deposits of amylopectin-like material. Differential diagnosis was made with basophilic degeneration of the myocardium, and with "polyglucosan bodies disease." In the existing literature we found only one case of juvenile amylopectinosis, and another four adult cases.