We describe a fatal case of infectious mononucleosis presenting with fulminant hepatic failure associated with extensive CD8-positive lymphocyte infiltration and diffuse karyorrhexis in the liver. Immunohistochemical analysis of mononuclear cells showed that Leu-2a (CD8)-positive lymphocytes were heavily distributed in the portal areas and the sinusoidal spaces, but Leu-3a (CD4)-, Leu-14 (CD22)-, or My 4 (CD14)-positive cells were undetectable in sections of the liver. Southern blot hybridization studies disclosed the presence of Epstein-Barr virus DNA fragments in the liver tissue. The unusual pathologic and immunologic responses observed in this case could not simply be explained by severe Epstein-Barr virus infection. Some superimposed factors should be considered.