TT virus (TTV) is a newly discovered virus from a patient with post-transfusion hepatitis. We investigated the frequency and pathogenesis of TTV infection in children. A semi-nested PCR assay was used to amplify TTV-DNA in serum samples from 254 ambulatory children without liver disease, 20 with hepatitis of unknown etiology, and 18 transfusion recipients or hemophiliacs. In positive samples, TTV-DNA was quantified by real-time quantitative PCR using a fluorescent probe. We detected TTV-DNA in 20% of children with hepatitis of unknown etiology, which was not statistically different from the 23% prevalence in ambulatory children. In transfusion recipients or hemophiliacs, the frequency was higher (50%) than that in ambulatory children (P = 0.01). Among ambulatory children, TTV-DNA was frequently detected in children with acute gastroenteritis (36%). TTV-DNA was detected in 10% of the infants under 6 months old, and 20% of the children from 7 to 12 months old. The prevalence was constant after the age of 1 year; however, the copy number of TTV-DNA was significantly higher in children under 1 year of age (mean: 10(5.4) versus 10(3.8) copies/ml, P= 0.008). Finally, TTV-DNA was quantified serially in three children with chronic hepatitis who were positive for TTV-DNA. The presence or amount of TTV-DNA was unrelated to the serum alanine aminotransferase level. These results indicate that TTV infection is common in children. The larger quantity of TTV-DNA in infants and the high prevalence of TTV in children of all ages suggest that TTV may be transmitted in early childhood. Its relationship to hepatitis is doubtful in children.