Recently, we identified TTV isolates from nonhuman primates and named them simian TTV (s-TTV). To investigate the prevalence of s-TTV in humans, we examined sera from healthy individuals and patients with liver diseases in Japan for the presence of s-TTV DNA by PCR assay. s-TTV DNA was determined by nested PCR using s-TTV-specific primers designed from untranslated region of s-TTV genome. s-TTV DNA sequence was detected in three of 200 (1.5%) healthy adults but none of 48 infants without liver disease. On the other hand, s-TTV DNA was detected in 30 of 287 (10.5%) Japanese patients with liver disease. s-TTV coinfection with hepatitis B virus and hepatitis C virus were present in 16.7 and 30% of these patients, respectively, while 53.3% of patients were positive for s-TTV alone. Nucleotide sequence analyses in 20 patients confirmed that these PCR products were derived from s-TTV genome sequences and nearly 85% identical to those of s-TTV prototypes from chimpanzees. Phylogenetic analysis demonstrated that all s-TTV isolates from humans were distinguished clearly from the human TTV isolates. Furthermore, s-TTV in humans was classified into two different genotypes as well as simians. Our results indicate that generally 10.5% of Japanese patients with liver diseases were infected with s-TTV. The routes of s-TTV transmission from animal to human require clarification.