Pompe disease (glycogen storage disease type II (GSD-II)) is a myopathy caused by a genetic deficiency of acid α-glucosidase (GAA) leading to lysosomal glycogen accumulation causing muscle weakness, respiratory insufficiency and death. We previously demonstrated in GSD-II mice that a single injection of a helper-dependent adenovirus (HD-Ad) expressing GAA resulted in at least 300 days of liver secretion of GAA, correction of the glycogen storage in cardiac and skeletal muscles and improved muscle strength. Recent reports suggest that gene therapy modeling for lysososomal storage diseases in mice fails to predict outcomes in larger animal models. We therefore evaluated an HD-Ad expressing GAA in non-human primates. The baboons not only tolerated the procedure well, but the results also confirmed that a single dose of the HD-Ad allowed the livers of the treated animals to express and secrete large amounts of GAA for at least 6 months, at levels similar to those achieved in mice. Moreover, we detected liver-derived GAA in the heart, diaphragm and skeletal muscles of the treated animals for the duration of the study at levels that corrected glycogen accumulation in mice. This work validates our proof-of-concept studies in mice, and justifies future efforts using Ad-based vectors in Pompe disease patients.