GSD type II, also known as acid maltase deficiency or Pompe disease, is a prototypic lysosomal disease. Its clinical presentation clearly differs from other forms of GSD. Deficiency of a lysosomal enzyme, alpha-1,4-glucosidase, causes GSD type II. Pompe initially described the disease in 1932. An essential pathologic finding is the accumulation of normally structured glycogen in most tissues. Three forms of the disease exist: infantile, juvenile, and adult. In the classic infantile form, the main clinical signs are cardiomyopathy and muscular hypotonia. In the juvenile and adult forms, the involvement of skeletal muscles dominates the clinical presentation. The images below illustrate histologic findings of GSD type II.