We report a 13-year-old boy with multisystem involvement secondary to accumulation of amylopectin-like material. He was born to consanguineous parents at full term without any complications and his maternal perinatal history was uneventful. His parents were cousins. He had normal growth and development except for his weight. His sister died from an unexplained cardiomyopathy at the age of 8 years. Our patient's initial symptom was severe heart failure. Since he also had a complaint of muscle weakness, electromyography was performed which showed muscle involvement. The diagnosis was suggested by tissue biopsy of skeletal muscle showing intracellular, basophilic, diastase-resistant, periodic acid-Schiff-positive inclusion bodies and was confirmed by the presence of a completed branching enzyme deficiency. Similar intracytoplasmic inclusion-like bodies were also found in liver biopsy, but very few in number compared with the skeletal muscle. The patient died from an intercurrent infection. Postmortem endomyocardial biopsy revealed the same intracytoplasmic inclusions as described above affecting almost all myocardial cells. Ultrastructural examination of liver biopsy was nondiagnostic; however, myocardium showed prominent, large, intracytoplasmic deposits. Glycogen branching enzyme gene sequence was normal, and thus classical branching enzyme deficiency was excluded. Our patient represents the first molecular study performed on a patient in whom there was multiple system involvement secondary to accumulation of amylopectin-like material. We suggest that this is an as yet undefined and different phenotype of glycogen storage disease associated with multisystemic involvement.