The hepatitis delta virus (HDV) is a defective virus dependent for replication and infection on helper functions provided by the hepatitis B virus (HBV). HDV and HBV co-infection occurs mainly in acute hepatitis patients with frequent parenteral exposure such as intravenous drug addicts and transfused patients. In contrast, co-infection is very rare in patients with other sources of infection and usually is a self limited disease. HDV superinfection of HBsAg carriers may have a severe course and proceed to chronic active liver disease. Active immunoprophylaxis against HBV should be considered as the only preventive measure against HDV infection.