We examined dystrophin, the protein product of the Duchenne muscular dystrophy gene, in muscle biopsy specimens from 4 male patients with quadriceps myopathy, all of whom showed a mild and slowly progressive myopathy confined to the quadriceps muscles. All 4 patients had clear abnormalities of dystrophin, and were diagnosed as having Becker muscular dystrophy by both immunofluorescence and immunoblot examinations; that is, dystrophin of an abnormal molecular mass was visualized in muscle cryosections as "patchy" or discontinuous immunostaining at the surface membrane of the muscle fibers. One patient had a brother who showed widespread myopathic changes consistent with typical Becker muscular dystrophy. We conclude that the syndrome called quadriceps myopathy includes a group of forme fruste Becker muscular dystrophy.