Gross pathologic lesions and light microscopic and ultrastructural features of skeletal muscle lesions in canine X-linked muscular dystrophy (CXMD) were studied in dogs from 3 months to 6 years of age. Necrosis and regeneration were present at all ages, but were most prominent in the youngest dogs studied. Increased intracytoplasmic calcium, as evidenced by positive alizarin red S staining, was associated with fiber necrosis, but was also seen in small numbers of otherwise normal fibers. Progressive changes included development of severe fiber size variation, endomysial and perimysial fibrosis, prominent cytoplasmic disorganization, internalization of myonuclei, mitochondrial proliferation, mild fat infiltration, and alterations in the fiber-type pattern. The most consistent early ultrastructural changes were dilatation of the sarcoplasmic reticulum and focal subsarcolemmal areas of degeneration. Convincing sarcolemmal defects were not found. Z-band streaming was present at all ages, and Z-band duplication and nemaline rods were seen in older dogs. Evidence for abnormal regeneration was found in the oldest dog, and was associated with extensive fibrosis. These findings document the progression of lesions in CXMD, and illustrate the profound alterations in fiber organization and fiber type that may occur in late stages of dystrophin-deficient muscular dystrophy.