Although the liver is the main target for hepatitis C virus (HCV) infection, HCV RNA of positive and negative polarity has also been detected in peripheral blood mononuclear cells (PBMCs) by polymerase chain reaction. However, no data have been published on the relationship between the number of HCV-infected PBMCs and serum viremia levels. To address this issue, PBMC samples from 20 patients with chronic hepatitis C were examined by fluorescent "in situ" hybridization. Serum viremia levels and viral load in infected PBMC were measured using the Amplicor Monitor test. HCV was detected in all PBMC samples corresponding to the HCV-positive patients. Fluorescent signals were found mainly in the cytoplasm of the cell. The percentage of positive cells ranged from 0.08% to 4%, with a statistical correlation with the viral load in PBMC (r = 0.69; p =. 001) but not with the serum viremia levels (r = 0.23). It was demonstrated that HCV infection of PBMCs is a common feature of HCV chronic carriers. The results suggest that HCV infection of PBMCs does not contribute significantly to HCV viremia.