Recent studies suggest that the seroprevalence of hepatitis C virus antibodies (HCVAb) among sexual partners of those who are HCVAb positive is higher than in the general population. Moreover some studies seem to indicate that transmission of hepatitis C virus (HCV) occurs more readily when there is HIV infection in the couple. We studied the prevalence of HCVAb seropositivity among the regular sexual partners of 84 HCVAb positive intravenous drug users (IVDUs), by means of ELISA confirmed by radio immuno-adsorbent assay, 11 generation (RIBA II). The couples were subdivided into three groups: group 1: 30 HIV negative IVDUs and their HIV negative partners (HIV-/HIV-); group 2: 47 HIV positive IVDUs and their HIV negative partners (HIV+/HIV-); group 3: seven HIV positive IVDUs and their HIV positive partners (HIV+/HIV+). The seroprevalence of HCVAb among the partners of IVDUs was 28.6% in the couples of group 3, 12.8% in the couples of group 2 and no partner was positive among the couples of group 1. There was no statistically significant difference in HCV transmission between the couples who never used a condom and those who always used one. The couples of group 1 never used a condom. We found HCVAb seropositivity only in the partners who were in couples in whom HIV was present. We did not demonstrate that sexual intercourse is a means of HCV transmission because none of the HIV-/HIV- couples of group 1 used a condom and no partner was HCVAb positive.(ABSTRACT TRUNCATED AT 250 WORDS)