BACKGROUND: The clinical significance of TT virus (TTV) coinfection in chronic hepatitis C (CHC) patients and influence of TTV viremia on hepatitis C virus (HCV) response to high dose interferon-alpha therapy in Taiwan were investigated. MATERIALS AND METHODS: Total 102 HCV RNA-positive CHC patients were enrolled. TTV DNA (using polymerase chain reaction primers derived from 5' non-coding region and open reading frame 2), alanine aminotransferase (ALT), GB virus-C/hepatitis G virus (GBV-C/HGV) RNA, anti-E2 antibody, genotype and RNA levels of HCV were tested. RESULTS: The prevalence of TTV DNA was 51.0%. The mean age of TTV viremic CHC patients was significant higher than non-viremic ones (P<0.05). HCV sustained viral response (SVR) was achieved in 42 (41.2%) patients. Based on multivariate regression analyses, SVR were significantly associated with low pretreatment HCV RNA levels, HCV genotype non-1b and high pretreatment levels of ALT but not TTV viremia. CONCLUSIONS: TTV viremia is highly prevalent among Taiwanese CHC patients and related to increased ages. Neither severity of liver disease nor replication and genotype distribution of HCV was affected by concurrent TTV infection. With high HCV SVR rate associated with pretreatment HCV RNA and ALT levels and HCV genotype, TTV viremia did not influence the HCV response.