Glycogen Storage Disease Type II is caused by the deficiency of acid maltase resulting in lysosomal accumulation of glycogen. There are two major clinical syndromes, a severe generalized and invariable fatal disease of infancy, and a myopathy starting in juvenile or adult life.

The clinical and laboratory findings of a patient with Glycogen Storage Disease Type II are presented. The patient, a 17-year-old male, experienced slowly progressive weakness of muscle of the pelvis shoulder girdles and trunk. Muscle biopsy showed vacuolar myopathy and electromyograph showed features of myopathy with fibrillation potentials, positive sharp waves, myotonic discharges, without clinical myotonia at rest, and polyphasic potentials on volition.

Clinical features, histopathologic and electrophysiologic findings of this disease and differential diagnosis were reviewed.