This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Glycogen Storage Disease type II (GSD-II) is a rare autosomal recessive disease caused by the deficiency of acid ?-glucosidase (GAA), which is needed for the degradation of lysosomal glycogen. Other names for GSD-II include Pompe Disease, acid maltase deficiency (AMD) and glycogenesis type II. Clinical presentation of GAA deficiency ranges from a rapidly fatal infantile disease to a slowly progressive late-onset myopathy frequently associated with respiratory insufficiency. (1)There is currently, no approved, effective treatment for GSD-II. Palliative and supportive care provides the mainstay of management. Enzyme replacement therapy may be effective in slowing or reversing symptoms of the disease or converting a more severe phenotype into a milder phenotype. It is hoped that enzyme replacement therapy will restore enzymatic activity, deplete accumulated substrate, and prevent further accumulation. Eligible patients in this study will receive an intravenous (IV) infusion of rhGAA of 20mg/kg of body weight every other week, for 52 weeks, and then will participate in a maintenance phase that lasts until the study is terminated or until market approval.