The localization of dystrophin was studied using the immunohistochemical method of diagnostic muscle specimens from 68 patients, aged 9 days to 65 years, with various neuromuscular disorders. Additionally muscle specimens from 2 normal humans and 2 normal mice were used as positive controls, and those from 2 mice with x-linked muscular dystrophy as negative controls. The specimens from all 14 Duchenne muscular dystrophy (DMD) patients, including one with preclinical DMD, showed negative dystrophin staining except for two which had 0.2% to 0.8% positive fibers. The mdx mice also showed negative dystrophin staining. In Becker muscular dystrophy (BMD), muscle fibers stained in a patchy or discontinuous fashion. Two symptomatic DMD carriers exhibited a distinct mosaic pattern of dystrophin positive and negative fibers. In contrast, dystrophin was present in all 7 biopsies from patients with 4 other types of muscular dystrophy (limb-girdle, congenital, myotonic and facioscapulohumeral). Other specimens, those from normal humans and control mice, revealed homogeneous immunostaining along the surface membranes of all muscle fibers. We thus conclude that immunohistochemical dystrophin staining can aid in differentiating DMD from preclinical DMD or BMD, as well as in the detection of DMD carriers.