In order to investigate a possible interaction between HIV and HCV infections, we compared HCV RNA levels in 29 matched pairs of haemophilic patients seropositive for HCV and serodiscordant for HIV. Levels were assayed using the new Chiron Quantiplex bDNA assay and were found to be significantly higher in HIV seropositive patients. There was no association between HCV RNA and age, duration of HCV infection, concentrate usage, markers of HIV progression, or use of zidovudine. Our study supports the hypothesis that HIV infection facilitates HCV replication and leads to more severe liver damage.