Duchenne muscular dystrophy (DMD) is an X-linked, lethal muscle disorder caused by mutations in the dystrophin gene. Although an adeno-associated virus (AAV) vector-mediated gene transfer provides an attractive approach to the treatment of DMD, limitation in insertion size up to 4.9 kb excludes incorporation of a full-length dystrophin cDNA (14 kb) into an AAV vector. We previously generated micro-dystrophin transgenic dystrophin-deficient mdx mice. In 4.9 kb rod-truncated micro-dystrophin CS1 transgenic mdx mice, dystrophic phenotypes were ameliorated almost completely (Biochem Biophys Res Commun 2002; 293:1265-72). We therefore constructed an AAV vector expressing micro-dystrophin CS1 driven by a skeletal muscle-specific MCK promoter, as the expression of the LacZ gene driven by the MCK promoter is longer in an AAV vector than in the CMV promoter in the skeletal muscle (Gene Ther 2002; 9:1576-88). We injected the AAV-MCK delta CS1 into the anterior tibial (TA) muscles of 5-week-old mdx mice, which exhibit active cycles of muscle degeneration/regeneration. At 8 weeks after the AAV vector injection, a large percentage of fibers were dystrophic-positive (10 to 50%). Even 24 weeks after injection, 15 to 75% of myofibers expressed micro-dystrophin. Dystrophin-positive fibers often had centrally located nuclei in the mice, however, the ratio was significantly reduced compared with that of dystrophin-negative fibers. We also measured tetanic force of AAV-MCK delta CS1-treated and non-treated mdx TA to evaluate functional amelioration. Non-treated mdx TA muscles showed marked reduction of specific tetanic force, while AAV-injected muscles showed moderate improvement. In conclusion, our study demonstrated that introduction of delta CS1 micro-dystrophin with an AAV vector successfully protected mdx muscles from progressive dystrophic degeneration.