Hemophilia B is an excellent candidate for gene therapy because low levels of factor IX (FIX) (≥1%) result in clinically significant improvement of the bleeding diathesis. Helper-dependent adenoviral (HDAd) vectors can mediate long-term transgene expression without chronic toxicity. To determine the potential for HDAd-mediated liver-directed hemophilia B gene therapy, we administered an HDAd expressing hFIX into rhesus macaques through a novel and minimally invasive balloon occlusion catheter-based method that permits preferential, high-efficiency hepatocyte transduction with low, subtoxic vector doses. Animals given 1 × 10(12) and 1 × 10(11) virus particle (vp)/kg achieved therapeutic hFIX levels for the entire observation period (up to 1,029 days). At 3 × 10(10) and 1 × 10(10) vp/kg, only subtherapeutic hFIX levels were achieved which were not sustained long-term. Balloon occlusion administration of HDAd was well tolerated with negligible toxicity. Five of six animals developed inhibitors to hFIX. These results provide important information in assessing the clinical utility of HDAd for hemophilia B gene therapy.