In patients chronically infected by hepatitis C virus (HCV), peripheral blood mononuclear cells (PBMCs) were shown to be targets for virus replication and in those coinfected with HIV, HCV viraemia was considerably increased. The purpose of this study was to quantify HCV RNA in PBMCs from 25 patients infected by HCV and from 25 patients coinfected by HCV and HIV. We used the branched DNA assay after extraction of total RNA on 5 x 10(6) cells to quantify HCV RNA, and the Inno LiPA assay to determine the HCV genotype. HCV RNA in PBMCs could be quantified in 8/25 patients in each group, but the HCV RNA concentration was very low in comparison with viraemia, since the highest result was 8.1 x 10(4) Eq genome/10(6) cells. In 10 ml of total blood, there was approximately 100 to 5,000 times less HCV RNA in PBMCs than in the plasma. It is therefore likely that PBMCs play only a minor part in the viral load present in the plasma. There was no preferential genotype associated with quantifiable HCV RNA in the PBMCs. In the case of HIV coinfection, there was no increase in the HCV-RNA concentration in PBMCs that could explain the increased viraemia observed in these patients. On the contrary, HCV RNA could not even be detected by RT-PCR in some of our coinfected patients.