The etiology of fulminant viral hepatitis was determined in 65 consecutive cases among 1,814 (3.58%) adults with acute viral hepatitis who were admitted to the Infectious Diseases Hospital of Athens from May, 1981 to August, 1983. Radioimmunoassays were used to detect hepatitis B virus, hepatitis A virus and delta-agent markers. Enzyme immunoassay was used to detect anti-HBc IgM. Hepatitis B virus was responsible for 48 (73.9%), non-A, non-B for 16 (24.6%) and hepatitis A virus for one case (1.5%). The use of anti-HBc IgM identified 10 cases of fulminant type B hepatitis among HBsAg negative, anti-HBc and anti-HBs positive patients. Fulminant non-A, non-B hepatitis was superimposed on 10 asymptomatic HBsAg carriers; delta-agent co-infection was diagnosed in two fulminant type B hepatitis cases. These data show that hepatitis B virus is the major cause of fulminant hepatitis in Greece. HBsAg carriers are at high risk for fulminant non-A, non-B hepatitis. delta-agent does not seem to be as important as in Italy and other countries.