Helper-dependent adenoviral vectors (HDAd) are attractive vectors for liver-directed gene therapy because they can mediate sustained, high level transgene expression with no long-term toxicity. However, as a consequence of a threshold effect, high vector doses are required to achieve efficient hepatic transduction by peripheral intravenous injection which unfortunately results in a dose-dependent activation of the innate inflammatory response. Clearly strategies to overcome the threshold to efficient hepatic transduction are needed to improve the therapeutic index of HDAd. We have recently shown that systemic hydrodynamic injection of HDAd into mice resulted in exceedingly high levels of hepatic transduction and reduction in both systemic vector dissemination and acute proinflammatory cytokines compared to conventional injection. Hydrodynamic injection offers a method of dramatically increasing the therapeutic index of HDAd but, unfortunately, cannot be applied to larger animals due to the large injection volume. Therefore, we have developed a minimally invasive method to mimic hydrodynamic injection in nonhuman primates which does not require injection of large volumes. This method involves the use of balloon occlusion catheters percutaneously positioned in the vena cava to transiently obstruct hepatic venous outflow to increase the intrahepatic pressure prior to simple peripheral intravenous injection of HDAd in a small volume. We demonstrate in nonhuman primates that this novel method of vector delivery results in an exceedingly high and unprecedented level of hepatic transduction with low vector doses resulting in stable, long-term transgene expression. This was accompanied by reduced systemic vector dissemination and minimal, transient elevations of proinflammatory cytokines. Despite the relatively high intrahepatic pressures achieved, there was minimal evidence of liver injury; no histological abnormalities were observed and only minimal and transient elevation of AST and ALT were noted. The procedure itself was uneventful and very well tolerated with no clinical manifestations of acute or chronic toxicities. This novel method of delivering HDAd is simple, minimally invasive, and clinically relevant since it increases the therapeutic index of HDAd for liver-directed gene therapy by allowing high efficiency hepatic transduction with low vector doses and with negligible acute or chronic toxicities.