Previously, we demonstrated that intrahepatic upregulation of the immunoactivating molecules CD40 and CD40 ligand (CD40L) are early mechanisms for liver cell damage in human and murine fulminant hepatic failure (FHF). In the present study, we investigated the functional effects of intrahepatic overexpression of CD40L by adenoviral-mediated gene transfer (AdCD40L) in mice. AdCD40L injection induced severe liver cell damage, which was associated with increased alanine aminotransferase (ALT) levels peaking at day 5 after vector administration (AdCD40L, 1,707 ± 279 U/L; AdLacZ, 213 ± 25 U/L) and with lethality in half of the mice. Except for mild splenomegaly, no organs other than the liver were involved in inflammatory reactions. CD40–CD40L interaction was mandatory for liver damage, because CD40−/− mice were completely protected. Furthermore, CD40L-induced FHF depended on competent lymphocytes, because inflammatory reactions were strongly decreased in SCID and Rag1−/− mice. In contrast, neither natural killer T (NKT) cells nor Kupffer cells relevantly influenced histology as shown in NKT cell–deficient CD1d−/− mice and by gadolinium depletion of Kupffer cells. Furthermore, immunosuppression by dexamethasone and cyclosporin A was not sufficient to block CD40L damage.In conclusion, we present a model of FHF with strong similarities to human FHF with respect to time course and histological changes. This model suggests involvement of the CD40–CD40L system in FHF and might have important implications for future pathophysiological studies of this condition.