Since its chance discovery a mere decade ago, the delta agent has been characterized as a novel pathogen that poses the risk of developing into a scourge of modern times. With its unique single-stranded circular RNA, exclusive dependence on HBV for its replication, and characteristic ability to suppress hepatitis B synthesis, the delta agent has emerged as an important global cause of fulminant hepatitis and progressive liver disease. Persons at greatest risk of delta infection are those living in endemic areas and HBsAg-carrier parenteral drug abusers, hemophiliacs, hemodialysis patients, and homosexual men in nonendemic areas. Widespread dissemination appears possible. There is as yet no known effective treatment for established delta infection, and, for the present, clinicians must concentrate on preventing HDV infection by incorporating vigorous use of the hepatitis B vaccine into strategies for preventing HBV infection.