Pompe disease is a lysosomal storage disorder caused by acid alpha-glucosidase deficiency and characterized by progressive muscle weakness. Enzyme replacement therapy (ERT) has ameliorated patients' perspectives, but reversal of skeletal muscle pathology remains a challenge. We studied pretreatment biopsies of 22 patients with different phenotypes to investigate to what extent fiber-type distribution and fiber-type-specific damage contribute to clinical diversity. Pompe patients have the same fiber-type distribution as healthy persons, but among nonclassic patients with the same GAA mutation (c.-32-13T>G), those with early onset of symptoms tend to have more type 2 muscle fibers than those with late-onset disease. Further, it seemed that the older, more severely affected classic infantile patients and the wheelchair-bound and ventilated nonclassic patients had a greater proportion of type 2x muscle fibers. However, as in other diseases, this may be caused by physical inactivity of those patients.