Using immunologic techniques, we studied vinculin, a cytoskeletal protein associated with the membrane-skeleton of the muscle fiber. We examined muscle biopsies from five patients with Duchenne muscular dystrophy (DMD), two with Becker's muscular dystrophy (BMD), three normal human muscle samples, and four biopsies from disease control patients. All DMD patients showed patchy and low-intensity immunostain at the sarcolemma of most fibers and, by immunoblot analysis, the content of vinculin was 42 to 61% of control values. There was no significant vinculin deficiency in samples from patients with BMD and other disease controls. The data suggest that vinculin content is reduced only in muscle where dystrophin is absent or sparse.