Of 377 cases of fulminant hepatitis in persons positive for hepatitis B surface antigen (HBsAg) in Greece, Italy, the United States, the United Kingdom, the Central African Republic, Taiwan, Egypt, and India, only 52% could be attributed to infection with hepatitis B virus, which was defined as the presence of the IgM antibody to the hepatitis B core antigen (IgM anti-HBc) and the absence of serum markers of infection by extraneous viruses. Thirty percent of cases were caused by coinfection with hepatitis B virus and hepatitis delta virus or by infection with hepatitis delta virus superimposed on carriers of chronic HBsAg. In 18% of the patients, the absence of IgM anti-HBc indicated that they were not known to carry HBsAg, but no obvious superimposed factor of hepatitis could be identified. The cause of fulminant hepatitis is complex, and major risk factors are a pre-existing HBsAg state and hepatitis delta virus infection. Superinfection of HBsAg carriers by non-A, non-B viruses seems to be the cause in a consistent proportion of cases.