Recently, an infection caused by a novel TT-virus (TTV), which was initially associated with post-transfusion non-A-G hepatitis, led us to investigate the TTV infection (clinic-epidemiological aspects, phylogenetic characterization and distribution of TTV genotypes) in different groups from São Paulo.The study included (i.) 138 children without history of blood transfusion or of liver disease, (ii.) 401 blood donors and (iii.)130 Patients with Bleeding Disorders (PBD).TTV infection was investigated by PCR-based amplification of the untranslated region (UTR) and its prevalence was as high in low-risk groups (81-85%) as in PBD (98%).The distribution of this infection according to the age groups only showed significant difference in the children group.Almost all the children aging 2 to 4 years (95%) were infected by TTV.However, this prevalence decreased continuously thereafter and reached a lower prevalence of 47% in adolescents of male gender aging 15 to 17 years.TTV DNA was also detected by a hemi-nested PCR amplification directed to the N22 region in blood donor and PBD samples; sequencing