The clinical profile and histopathologic changes in needle biopsies of the liver were studied in 10 cases of acute Epstein-Barr virus infection occurring in liver transplant recipients. The systemic viral syndrome in four cases resembled that seen in infectious mononucleosis, whereas in six others it was characterized by atypical signs and symptoms in the form of jaw pain, arthralgias, joint space effusions, diarrhea, encephalitis, pneumonitis, mediastinal lymphodenopathy, and ascites. Laboratory investigation showed marked elevations in hepatocellular enzymes and circulating atypical lymphocytes in the peripheral blood. Pancytopenia was noted in eight cases. A range of histopathologic changes was noted in the allografts ranging from alterations typically observed in infectious mononucleosis to a distinctive constellation characterized by (a) mixed mononuclear portal and sinusoidal infiltrates containing atypical large noncleaved cells and immunoblasts; (b) associated lobular activity indicative of a hepatitic process, and (c) relatively mild duct damage not in proportion to the severity of the inflammatory infiltrates. The patients responded to reduced immunosuppression, but recurrent viral syndromes occurred in four instances and one patient died of systemic lymphoproliferative disease.