The distribution of cathepsin D, an acidic endopeptidase, was localized by immunocytochemistry in human skeletal muscle obtained from 34 persons with a variety of neuromuscular disorders. Normal human skeletal muscle contained small amounts of cathepsin D, all of which was found close to the sarcolemmal membrane. Immunoreative cathepsin D was present in the cytoplasm of many infiltrating phagocytic cells and was increased in skeletal muscle fibers from patients with muscular dystrophies, inflammatory myopathies, rhabdomyolysis, acid maltase deficiency, and neurogenic atrophy. In cases of Duchenne type muscular dystrophy, the increase in cathepsin D was especially prominent in small regenerating fibers, in which it was visualized at the ultrastructural level in lysosome-like organelles anextralysosomal locations. The function of cathepsin D in skeletal muscle is unclear, but the present findings suggest a possible role in muscle regeneration and repair. Such a role would necessitate careful selection of drugs which interfere with proteolytic activity if they are to be used as therapeutic agents in treating neuromuscular diseases.