Five adult patients presenting with clinical and laboratory manifestations of an acute hepatitis in the course of a hitherto undiagnosed infectious mononucleosis (IM) are reviewed. Chief complaints were intense malaise and prolonged fever (7 to 15 days prior to diagnosis). Serum aminotransferases were moderately raised in all patients; three patients had mild jaundice with a direct-reacting hyperbilirubinemia; 4 patients had an enlarged and tender liver. When making the differential diagnosis of causes of acute hepatitis, blood smear examination was crucial, showing atypical lymphocytes (Downey). The diagnosis of IM was confirmed by the demonstration of high serum titers of antibodies against Epstein-Barr virus, IgM class (4 patients) or heterophil antibodies (1 patient), plus peripheral lymph node enlargement (3 patients), splenomegaly (4 patients) and the time course of the disease. The relevance of blood smear examination as a practical tool in the diagnosis of causes of acute hepatitis is stressed.