Duchenne muscular dystrophy (DMD) and murine X-linked muscular dystrophy (mdx) are both due to absence of the subsarcolemmal protein dystrophin. Recombinant adenovirus vectors (AdV) are considered a promising means for delivering a functional dystrophin gene to muscle. However, the usefulness of AdV for this purpose is limited by vector toxicity as well as immune-mediated elimination of infected fibers. In addition, studies to date of AdV-mediated dystrophin gene transfer have either failed to examine effects on muscle strength or been performed in immunologically immature neonatal animals with little baseline abnormality of force-generating capacity. In the present study, AdV-mediated dystrophin gene transfer was performed in adult mdx mice with pre-existent dystrophic pathology and muscle weakness. The main findings are as follows: (1) acute myofiber toxicity and gene transfer efficiency are both AdV dose-dependent, such that the therapeutic margin of safety is fairly narrow; (2) immunosuppressive therapy (FK506) prevents immune-mediated elimination of dystrophin-positive fibers but not the dose-dependent toxic effects; (3) at the optimal vector dosage and with effective immunosuppression, AdV-mediated dystrophin minigene transfer is capable of alleviating the loss of force-generating capacity as well as histopathological evidence of disease progression normally seen in adult mdx muscles over a 2-month period. These findings have important implications for the eventual application of AdV-mediated dystrophin gene transfer in DMD patients.