To characterize the natural progression of infantile-onset Pompe disease. Retrospective chart reviews of 168 patients with documented acid alpha-glucosidase deficiency and symptom onset by 12 months of age; Kaplan-Meier analysis of total and ventilator-free survival time; Cox proportional hazards regression modeling of mortality risk factors. The median age at symptom onset was 2.0 months (range 0 to 12 months), 4.7 months at diagnosis (range: prenatal to 4.2 months), 5.9 months at first ventilator support (range 0.1 to 31.1 months), and 8.7 months at death (range 0.3 to 73.4 months). Survival rates at 12 months of age were 25.7% overall and 16.9% ventilator-free; at 18 months 12.3% and 6.7%. Cardiomegaly (92%), hypotonia (88%), cardiomyopathy (88%), respiratory distress (78%), muscle weakness (63%), feeding difficulties (57%), and failure to thrive (53%) appeared after a median age of approximately 4.0 months. Multiple covariate analysis confirmed that early symptom onset increased risk of early death. Despite frequent therapeutic interventions, infantile-onset Pompe disease remains lethal.