Abstract Infections by the hepatitis B or C virus are extremely common causes of acute and chronic liver disease, and coexistence of the two viruses in the same patient is not rare. Evidence has been found that such interaction may play an important role in fulminant hepatitis and in the development of hepatocellular carcinoma in cirrhotic patients. Liver disease activity and prognosis have been reported to be generally more serious in the presence of double infection, although an inverse relationship in the replicative levels of the two agents has been noted, suggesting viral interference, particularly in cases of chronic hepatitis. Thus, the two viruses seem to inhibit each other at the molecular level, while cytopathic effects appear to be enhanced. Further studies are needed to explain the mechanisms of these apparently contrasting effects.