The hepatitis D virus (HDV), also called delta virus, is a small circular RNA virus. The HDV is dependent on the hepatitis B virus (HBV) and can cause infection in normal individuals with hepatitis B or yet, superinfect chronic HBV carriers. Three genotypes have already been cloned and sequenced. Infection with HDV has a worldwide distribution and a high HDV endemicity has been documented in the western Amazon region, in Brazil. It has been estimated that 18 million people are infected with this virus amongst the 350 million carriers of the HBV around the world. The HDV transmission and risk factors for infection are similar to those for HBV infection. The diagnosis is based on the immunohistological identification of HDAg in the liver and detection of IgM and IgG anti-HD in serum using RIA or EIA. The clinical course of hepatitis D is variable. Fulminant disease occurs more commonly in hepatitis B and D than in other forms of acute viral hepatitis. Chronic HDV infection is usually associated with severe histological changes in the liver and with a rapidly progressive course, that can lead to cirrhosis, liver failure and death. Treatment of chronic hepatitis D is currently unsatisfactory and interferon alpha is the only agent found to have some effect on the course of chronic hepatitis. Orthotopic liver transplantation is indicated for terminal cases of cirrhosis. Prophylaxis for HDV infection is possible by vaccination against the hepatitis B virus.