The use of recombinant adeno-associated virus (AAV) virions for delivery of DNA molecules to muscle cells and tissue is disclosed. The invention allows for the direct, in vivo injection of recombinant AAV virions into muscle tissue, e.g., by intramuscular injection, as well as for the in vitro transduction of muscle cells which can subsequently be introduced into a subject for treatment. The invention provides for sustained, high-level expression of the delivered gene and for in vivo secretion of the therapeutic protein from transduced muscle cells such that systemic delivery is achieved.