Hepatitis C virus (HCV) is usually transmitted parenterally, but sexual transmission is considered likely in the 20% of cases with no other risk factors. Retrospective cohort studies conducted among persons who have never injected drugs show that factors predictive of HCV seropositivity include the number of lifetime sexual partners, high-risk sexual practices, other sexually transmitted infections, and HIV seropositivity. Persons in long-term monogamous heterosexual relationships with a partner seropositive for HCV are at lower risk of HCV acquisition (0 to 0.6% peryear) than persons with multiple partners or those at risk for sexually transmitted diseases (0.4 to 1.8% per year). HCV RNA is detectable in genital fluids, but there is not yet any proof that the HCV RNA in genital secretions represents infectious virus. HCV can be transmitted by sexual intercourse but much less efficiently than other sexually transmitted viruses, such as HBV and HIV. Sexual transmission of HCV may be enhanced by other concomitant sexually transmitted infections with genital erosive lesions or via traumatic sexual intercourse with abrasion of the genital mucosa.