A randomly selected extensor digitorum brevis (EDB) muscle in each of three Duchenne muscular dystrophy (DMD) boys aged 9 to 10 was injected with approximately 8 times 10₆ myoblasts. The contralateral EDBs were sham‐injected with carrier solution. Donor myoblasts were derived from cell culture of muscle biopsies from the normal ward or normal brothers of the recipients. Cyclosporine (CsA) treatment began two days before myoblast injection and continued for three months. Three days prior to myoblast injection and three months after, the isometric twitch and maximum voluntary contraction of the left and the right EDBs were measured. Myoblast‐injected EDBs showed increases in tensions whereas sham‐injected EDBs showed reductions. Both immunocytochemical staining and immunoblot revealed dystrophin in the myoblast‐injected EDBs. Dystrophic characteristics such as fiber splitting, central nucleation, phagocytic necrosis, variation in fiber shape and size, and infiltration of fat and connective tissues were less frequently observed in these muscles. Sham‐injected EDBs exhibited significant structural and functional degeneration and no dystrophin. Throughout the study, there was no sign of erythema, swelling or tenderness at the injection sites. Serial laboratory evaluation including electrolytes, creatinine, and urea did not reveal any significant changes before or after myoblast transfer. We conclude that myoblast transfer therapy is a safe and efficacious procedure to improve the biochemistry, structure, and function of degenerative EDB muscles in DMD.