Glucocorticoid therapy slows the progression of Duchenne muscular dystrophy. In muscle cultures, the addition of the glucocorticoid methylprednisolone increases myogenesis in most normal mixed and clonal cultures. Conversely, in some normal clonal and most dystrophic cultures, methylprednisolone inhibits fusion. However, in fusion-arrested normal and Becker muscular dystrophy cultures, dystrophin is expressed independently of fusion and of myosin heavy chain expression, and in some cases, expression is apparently enhanced by methylprednisolone. We suggest that dystrophin is a muscle-specific protein that does not require fusion for expression, and the methylprednisolone-induced enhancement of dystrophin expression may account for some of the clinical benefits of glucocorticoids in vivo.