Mice carrying mutations in both the dystrophin and utrophin genes die prematurely as a consequence of severe muscular dystrophy. Here, we show that intravascular administration of recombinant adeno-associated viral (rAAV) vectors carrying a microdystrophin gene restores expression of dystrophin in the respiratory, cardiac and limb musculature of these mice, considerably reducing skeletal muscle pathology and extending lifespan. These findings suggest rAAV vector-mediated systemic gene transfer may be useful for treatment of serious neuromuscular disorders such as Duchenne muscular dystrophy.