Transfusion transmitted virus (TTV) is a new DNA virus found in patients with post-transfusion hepatitis. The prevalence of this virus among dental patients has not been reported, therefore, the prevalence of TTV infection in consecutive dental inpatients was evaluated. TTV DNA was assayed by the polymerase chain reaction (PCR) in 441 dental inpatients with oral cancer (n=192) or oral cysts (n=249). The serum HBs antigen and HCV antibody as well as aspartate transaminase (AST), alanine transaminase (ALT), and gamma glutamyl transpeptidase (γ-GTP) concentrations were also measured. Of 441 subjects, 137 were infected with TTV (31.1%). This prevalence of TTV was much higher than that of HBV or HCV (HBV 1.2%; HCV 6.0%) in these dental patients. There was no gender or age difference in the prevalence of TTV infection. Of the 192 patients with oral cancer, 57 subjects had TTV in their sera, while 80 of 249 with oral cystic disease had TTV. The prevalence of TTV was similar between the two different disease groups. Neither the serum ALT nor serum AST concentrations were different between the subjects positive and negative for TTV DNA. In hospitalized dental patients, 31.1% were infected with TTV. The prevalence of TTV was much higher than that of HBV or HCV. There was no difference in the prevalence of TTV between subjects with cancer and cysts. Dentists should maintain high standards of infection control when treating any dental patient.