Fibrosis is a pathological feature observed in patients with Duchenne muscular dystrophy (DMD) and in mdx mice, the experimental model of DMD. We evaluated the effect of suramin, a transforming growth factor‐beta 1 (TGF‐β1) blocker, on fibrosis in mdx mice. mdx mice (6 months old) received suramin for 7 weeks. Suramin‐ and saline‐treated (control) mdx mice performed exercise on a treadmill to worsen disease progression. Immunoblotting showed an increase of TGF‐β1 in mdx diaphragm, limb, and cardiac muscles. Suramin decreased creatine kinase in mdx mice and attenuated fibrosis in all muscles studied, except for cardiac muscle. Suramin protected limb muscles against damage and reduced the exercise‐induced loss of strength over time. These findings support a role for TGF‐β1 in fibrinogenesis and myonecrosis during the later stages of disease in mdx mice. Suramin might be a useful therapeutic alternative for the treatment of dystrophinopathies. Muscle Nerve, 2011