TTV is a new virus which was identified in the serum of a patient with non-A-G post-transfusion hepatitis in Japan. The original workers aimed to account for the small number of post transfusion hepatitis cases found in their clinical practice. Subsequent work has attempted to determine the properties and unravel the natural history of the new agent. The original study applied representational difference analysis to detect foreign DNA sequences which were present only during the acute phase of illness. Subsequent studies have used PCR to study the agent in serum liver and faeces. This review summarises the published data from clinical and epidemiological studies in different countries. The inclusion of the virus in the parvovirus family seems premature because its size is unknown, its reported density is too light and its sequence lacks the characteristic long terminal repeats. The agent can be found in 1-40% of health blood donors in different countries and also in faeces. TTV is ubiquitous but its taxonomic identity and disease load remain to be determined.