The ultrastructural distribution of basement membrane around the capillaries as well as muscle cells of boys with Duchenne-type muscular dystrophy was determined. In dystrophic muscles, a diffuse thickness of vascular as well as muscular basement membrane was observed. Lamina densa lost its regular ribbon-like appearance and was split into several thin layers. After staining with tannic acid (TA), a densely stained meshwork was present on the muscle cell surface as well as in the extra-cellular space. Hyaluronidase treatment removed TA-stained deposits, indicating that hyaluronic acid is a major component. Enzyme resistant structures, presumably fibrous long-spacing collagen fibrils, have been found in dystrophic muscles. Based on the results of the study, faulty structure of the basement membrane in dystrophic muscle is suggested.