A patient with mucopolysaccharidosis type IIA (MPS IIA) and progressive gait disturbance is described. The histopathology of biopsied muscle was studied; Dorling's method revealed muscle fibers and interstitial cells containing metachromatic granules which suggested the storage of sulfated acidic glycosaminoglycans. Electron microscopy demonstrated that the membrane-bound vacuoles were present in muscle fibers, subsarcolemmal area, vascular endothelial cells, satellite cells, and endomysial fibroblasts. Besides clinical features, this ultrastructural pathology in MPS IIA muscles of MPS IIA was more severe than MPS IIB muscles. The accumulation of glycosaminoglycans in muscle tissue may be an additional factor contributing to gradual motor impairment of patients with MPS IIA.