TT virus (TTV) was first detected in the blood of three patients with elevated serum alanine aminotransferase following transfusion who were negative for all known hepatitis viruses. This virus exhibited hepatotropism, and its titers correlated with elevation in serum aminotransferase concentration suggesting that it was a true hepatitis virus. Moreover, it was demonstrated that the presence of TTV DNA is not associated with biochemical or histologic evidence of liver injury. The virus has been found worldwide with a high prevalence in the general population and there is evidence that it may be transmitted by parenteral exposure to blood, enterally and transmitted from mother to child. An association between TTV infection and acute or chronic hepatitis or other diseases has not been consistently observed.