Pompe disease, or glycogen storage disease type 2, is a rare inheritable metabolic disease caused by a deficiency of the lysosomal enzyme acid α-glucosidase. Patients with the classic infantile form of Pompe disease present with symptoms during the first 3 months after birth, and most will die within their first year. Recently, enzyme replacement therapy (ERT) with recombinant human α-glucosidase became commercially available for Pompe disease. This is a case report of an 8-year-old girl with the infantile form of Pompe disease who is one of the longest survivors through ERT. The patient was tetraplegic when she started ERT. At age 3 years, she developed massive gingival overgrowth and could not close her mouth, prompting a reduction of the gingival overgrowth surgically. We expected that massive accumulation of glycogen would explain the gingival overgrowth. However, histopathology of the gingiva tissue showed marked glycogen accumulation in smooth muscle cells of the arteries, but the glycogen content in fibroblasts did not exceed that of control individuals. Further, there was an increase of immature collagen in the connective tissue, and signs of a mild chronic inflammation. We concluded that glycogen storage is not a direct causative factor of gingival overgrowth in our patient. Chronic inflammation, dryness of the gingiva, or even the minimal glycogen accumulation in the fibroblasts may have played a role.