Glycogen storage disease type II (GSD II) is an autosomal recessive disorder caused by defects in the lysosomal acid alpha-glucosidase (GAA) gene. We investigated the feasibility of using a recombinant adenovirus containing the human GAA gene under the control of the cytomegalovirus promoter (AdCMV-GAA) to correct the enzyme deficiency in different cultured cells from patients with the infantile form of GSD II. In GAA-deficient fibroblasts infected with AdCMV-GAA, transduction and transcription of the human transgene resulted in de novo synthesis of GAA protein. The GAA enzyme activity was corrected from the deficient level to 12 times the activity of normal cells. The transduced cells overexpressed the 110 kDa precursor form of GAA, which was secreted into the culture medium and was taken up by recipient cells. The recombinant GAA protein was correctly processed and was active on both an artificial substrate 4-methylumbelliferyl-alpha-D-glucopyranoside (4MUG) and glycogen. In GAA-deficient muscle cells, a significant increase in cellular enzyme level, approximately 20-fold higher than in normal cells, was also observed after viral treatment. The transduced muscle cells were also able to efficiently secrete the recombinant GAA. Moreover, transfer of the human transgene resulted in normalization of cellular glycogen content with clearance of glycogen from lysosomes, as assessed by electron microscopy, in differentiated myotubes. These results demonstrate phenotypic correction of cultured skeletal muscle from a patient with infantile-onset GSD II using a recombinant adenovirus. We conclude that adenovirus-mediated gene transfer might be a suitable model system for further in vivo studies on delivering GAA to GSD II muscle, not only by direct cell targeting but also by a combination of secretion and uptake mechanisms.