The mechanism whereby dystrophin deficiency leads to excessive fibrosis and muscle degeneration is not known. The absence of dystrophin in skeletal muscle is associated with reduced plasma membrane stability as evidenced by elevated serum levels of the cytoplasmic enzyme creatine kinase. Basic fibroblast growth factor, a cytoplasmic polypeptide growth regulator that stimulates connective tissue synthesis, induces satellite cell proliferation, and suppresses myogenic differentiation, is made by skeletal muscle. We hypothesize that dystrophin deficiency leads to the constant release of basic fibroblast growth factor, which in turn contributes to fibrosis and muscle weakness by stimulating connective tissue and suppressing skeletal muscle differentiation. As an initial step in testing this hypothesis, we measured basic fibroblast growth factor in the serum of Duchenne muscular dystrophy patients. We found that whereas basic fibroblast growth factor was undetectable in the serum of normal individuals (n = 200), levels were elevated in 11 of 18 patients with Duchenne muscular dystrophy.