A major concern in therapy of acute liver failure is protection of hepatocytes to prevent apoptosis and maintain liver function. Small interfering RNA (siRNA) is a powerful tool to silence gene expression in mammalian cells. To evaluate the therapeutic efficacy of siRNA in vivo we used different mouse models of acute liver failure. We directed 21-nt siRNAs against caspase 8, which is a key enzyme in death receptor-mediated apoptosis. Systemic application of caspase 8 siRNA results in inhibition of caspase 8 gene expression in the liver, thereby preventing Fas (CD95)-mediated apoptosis. Protection of hepatocytes by caspase 8 siRNA significantly attenuated acute liver damage induced by agonistic Fas (CD95) antibody (Jo2) or by adenovirus expressing Fas ligand (AdFasL). However, in a clinical situation the siRNAs most likely would be applied after the onset of acute liver failure. Therefore we injected caspase 8 siRNA at a time point during AdFasL- and adenovirus wild type (Adwt)-mediated liver failure with already elevated liver transaminases. Improvement of survival due to RNA interference was significant even when caspase 8 siRNA was applied during ongoing acute liver failure. In addition, it is of particular interest that caspase 8 siRNA treatment was successful not only in acute liver failure mediated by specific Fas agonistic agents (Jo2 and AdFasL) but also in acute liver failure mediated by Adwt, which is an animal model reflecting multiple molecular mechanisms involved in human acute viral hepatitis. Consequently, our data raise hope for future successful application of siRNA in patients with acute liver failure.