Blood contamination has been proposed as TTV transmission. We studied the genoprevalence of TTV in Japanese men with history of intravenous drug abuse and/or tattoo. TTV was identified in serum by a polymerase chain reaction. TTV was detected in 89.7 percent of the men with history of intravenous drug abuse and/or tattoo, 74.4 percent of chronic hepatitis C patients, 78.0 percent of the chronic hepatitis B, and 65.8 percent of chronic hepatitis nonB nonC patients. Serum ALT levels of those infected with TTV alone were 27.2 +/- 17.5 IU/L. In the patients with chronic hepatitis C, serum ALT levels of those coinfected with TTV were similar to serum ALT levels of those without TTV infection. These results suggest that TTV causes no or mild hepatitis.