Gene delivery of IFN-alpha to the liver may represent an interesting strategy to maximize its antiviral efficacy and reduce side effects. We used a recombinant adeno-associated virus (AAV) encoding woodchuck IFN-alpha (AAV-IFN) to treat animals with chronic woodchuck hepatitis virus infection. The vector was given by intraportal or intramuscular route. Long-term transgene expression was detected after intraportal administration of an AAV encoding luciferase. In contrast, in the majority of the animals that received AAV-IFN through the portal vein, the expression of IFN-alpha was transient (30-40 days) and was associated with a significant but transient decrease in viral load. One animal, in which hepatic production of IFN-alpha persisted at high levels, died because of bone marrow toxicity. The disappearance of IFN-alpha expression correlated with the disappearance of AAV genomes from the liver. Intramuscular administration of AAV-IFN resulted in prolonged but fluctuating expression of the cytokine with no significant antiviral effect. In summary, this report shows that long-term expression of IFN-alpha in muscle is feasible but higher interferon levels might be needed to control viral replication. On the other hand, IFN-alpha gene delivery to the liver using an AAV vector induces a significant but transient antiviral effect in the woodchuck model.