We demonstrate that a single intraportal vein injection of a recombinant adeno-associated virus (rAAV) vector encoding canine factor IX (cFIX) cDNA under the control of a liver-specific enhancer/promoter leads to a long-term correction of the bleeding disorder in hemophilia B dogs. Stable expression of the therapeutic level of cFIX (5% of normal level) was detected in the plasma of a dog injected with an AAV vector at a dose of 4.6 × 10¹² particles/kg for over 7 months. Both whole-blood clotting time (WBCT) and activated partial thromboplastin time (aPTT) of the treated dogs have been greatly decreased since the treatment. No anti-canine factor IX antibodies have been detected in the treated animals. Importantly, no bleeding has been observed in the dog that expresses a therapeutic level of cFIX for 7 months following vector administration. Moreover, no persistent significant hepatic enzyme abnormalities were detected in the treated dogs. Thus, a single intraportal injection of a rAAV vector expressing cFIX successfully corrected the bleeding disorder of hemophilia B dogs, supporting the feasibility of using AAV-based vectors for liver-targeted gene therapy of genetic diseases.