Introduction: Torque teno virus (TTV) and SEN virus are circular single-stranded DNA viruses that cause blood-borne infections. The SEN virus (SEN-V) was originally detected in the serum of an injection drug user infected with human immunodefi ciency virus (HIV). Recently TTV was discovered as a potential causative agent of non-A-E hepatitis. The aim of this study was to investigate the prevalence of the SEN-V-D/H and TTV in HIV patients and healthy blood donors in Iran. Methods: One hundred and fi fty HIV patients with a mean age of 50.46 ± 18.46 years and 150 healthy blood donors with a mean age of 48.16 ± 13.73 years were included in this study. TTV and SEN-V were detected by the PCR and were quantitatively assayed by competitive PCR (nested and semi-nested PCR). Restriction fragment length polymorphisms (RFLPs) were used to determine the heterogeneity of TTV. Results: TTV and SEN-V were detected 96 (64%) and 84 (56%) of 150 HIV patients respectively. These rates were 34% (n=51) and 37.33% (n=56) in healthy blood donors (signifi cant, p<0.05). PCR detected SEN-V/TTV DNA from 32 of the healthy blood donors (21.33%), while 65 (43.33%) of HIV patients were positive for SEN-V/TTV DNA. Of 150 HIV patients, 32.66% and 23.33% were positive for SEN-V-H and SEN-V-D, respectively and 18.66% (n=28) were co-infected with SEN-V-D/H. Conclusions: The prevalence of SEN-VD/H and TTV is higher in HIV patients than in healthy blood donors in Southern Iran. Our results suggest that TTV and SEN-V might play a role in the development of liver disease in patients with immunodefi ciency diseases.Keywords: Torque teno virus. SEN virus. HIV. Genotype. Blood donors.Recently, a new virus, designated SEN virus (SEN-V), was isolated from serum of an human immunodefi ciency virus (HIV)-positive patient who was an injection drug user 1 . Nine different SEN-V genotypes have been identifi ed (A to I) which have at least 25% divergence in their nucleotide sequences 2 . The SEN-V-D and SEN-V-H genotypes are most frequently in patients with unknown hepatitis (non-A E hepatitis) but exhibit lower frequencies in the sera of healthy blood donors 3 . In addition Torque teno virus (TTV), also known as transfusiontransmitted virus was fi rst identifi ed in a Japanese patient in 1997 4,5 . Torque teno virus is a circular, single-stranded deoxyribonucleic acid (DNA) virus that chronically infects healthy individuals of all ages worldwide. Substantial data exists describing the prevalence and genetic heterogeneity of TTV in healthy populations and patients infected with HIV. The genome of SEN-V is similar to TTV and both are classifi ed within the circovirus family 5 . Both SEN-V and TTV are single-stranded non-enveloped DNA viruses of 3,800 nucleotides 6,7 . Several studies have shown that in HIV-infected patients the prevalence of TTV is slightly higher compare to healthy blood donors 8 . Puig-Basagoiti et al. described a high TTV prevalence among a large HIV-infected group in Spain 9 . The TTV prevalence of 76% among HIV positive individual ...