This is a longitudinal natural history study of Infantile Pompe disease. The investigators will regularly collect and review medical information regarding the diagnosis of Pompe disease, response to enzyme replacement (ERT) using alglucosidase alfa (Lumizyme/Myozyme) and response to immunosuppressive therapy in cases at risk for developing or those who have developed high and sustained antibodies to ERT. To follow the long-term outcomes, we will collect medical records including but not limited to the diagnosis, clinical parameters, assessments for clinical monitoring, and laboratory values including antibody testing results.