OBJECTIVES: Antibodies against herpes simplex viruses-1 and -2, cytomegalovirus, and syphilis were determined in six heterosexual couples with strong indications of having sexually transmitted hepatitis C virus infection and in 17 other heterosexual couples in which one partner was hepatitis C virus viremic (source partner), but the other had remained hepatitis C virus uninfected (exposed partner). STUDY DESIGN. Antibody testing was done with an enzyme-linked immunosorbent assay. Anti-herpes simplex virus 2 and anti-hepatitis C virus findings were further confirmed by immunoblotting. Hepatitis C virus RNA was determined by polymerase chain reaction and genotyped with type-specific primers. RESULTS. Five of six anti-hepatitis C virus-positive exposed heterosexual partners without parenteral risk factors, compared with three of 17 anti-hepatitis C virus-negative exposed partners, had antibodies to herpes simplex virus-2. On the other hand, no statistically significant difference was found regarding the frequency of herpes simplex virus-2 seropositivity when source partners in the anti-hepatitis C virus concordant and discordant couples were compared. The presence of antibodies to herpes simplex virus-1, cytomegalovirus, and syphilis did not significantly differ between source or exposed partners in anti-hepatitis C virus concordant and discordant couples, respectively. No predominance of any one hepatitis C virus genotype or liver morphology in couples concordant compared with discordant for anti-hepatitis C virus was found. CONCLUSIONS. The findings support the role of herpes simplex virus-2 in the heterosexual transmission of hepatitis C virus infections, and more specifically an increase in susceptibility to hepatitis C virus infections in exposed heterosexual partners with antibodies to herpes simplex virus-2