Hepatitis C virus (HCV), the major causative agent of non-A, non-B hepatitis, is transmitted by parenteral exposure. Transmission by sexual activity, however, is controversial. Possible behavioral risk factors for HCV infection were studied retrospectively among imprisoned men (n = 201, mean age: 45 years [S.D. 13]) who visited a health service center at a Japanese correctional facility for medical examination. Seropositivity of anti-HCV antibody was disproportionately high (49.8%) in comparison with volunteer blood donors. Among possible risk factors significant on univariate analysis, intravenous drug abuse and Tama-Ire, a Japanese custom of sexual behavior that suggests frequent, aggressive or promiscuous heterosexual activity, proved to be independent risk factors for HCV infection (odds ratio = 7.39, 95% CI = 3.41 - 16.05, P < 0.0001; odds ratio = 3.16, 95% CI = 1.16 - 8.64, P = 0.026, respectively) as shown by logistic regression analysis. The data suggest that HCV may be transmitted by sexual activity.