Systemic intravascular delivery of adenoviral (Ad) vectors for liver-directed gene therapy has been widely employed because of its simplicity, noninvasiveness, and potential for high transduction. For first-generation Ad vectors (FGAd), this results in high but transient levels of transgene expression and long-term hepatotoxicity due to viral gene expression from the vector backbone. Furthermore, high doses also result in an acute innate inflammatory response with potentially lethal consequences. Unlike FGAd, helper-dependent Ad vectors (HDAd) contain no viral genes and can provide sustained, high-level transgene expression with negligible long-term toxicity. However, whether the absence of viral gene expression leads to any decrease of acute toxicity in nonhuman primates has yet to be determined. To address this, we injected one baboon with 5.6 x 10(12) HDAd viral particles (VP)/kg and a second with 1.1 x 10(13) VP/kg. Approximately 50% hepatocyte transduction, accompanied by mild and transient acute toxicity, was observed in the animal receiving the lower dose. In the animal receiving the higher dose, 100% hepatocyte transduction, accompanied by lethal acute toxicity, was observed. These results indicate that systemic delivery of HDAd, like FGAd, results in acute toxicity in baboons consistent with activation of the innate inflammatory response, the severity of which is dose dependent, and confirm the hypothesis that Ad-mediated acute toxicity is independent of viral gene expression.