Identification of hepatitis C virus (HCV)-RNA in blood serum is crucial for hepatitis C diagnosis and for appropriate treatment. Detection of HCV-RNA in blood serum is used for therapy monitoring of patients with hepatitis C. Despite HCV-RNA elimination from blood serum during treatment in some patients, HCV viremia appears again after the completion of therapy. The aim of this study was to assess HCV-RNA in peripheral blood mononuclear cells (PBMCs) of hepatitis C patients in relation to HCV-RNA and antibodies to HCV in the serum. The study involved 71 patients undergoing anti-viral therapy (interferon and ribavirin). RNA isolated from serum and PBMCs was examined for the presence of HCV-RNA by an RT-PCR technique using specific oligonucleotide primers or by commercially available kits. In order to show the possible presence of HCV sequences in PBMCs, molecular DNA probes were constructed with a PCR amplicon and biotin-labelled by nick translation, and FISH and extended chromatin fibers in situ hybridization (ECFs-FISH) techniques were used. A 24-month follow-up study revealed that 34 out of 59 patients (58%) eliminated HCV-RNA from their sera. In the serum negative group, HCV-RNA was detected in PBMCs of 2 patients. The presence of HCV-RNA in PBMCs was confirmed by the FISH technique. In the ECFs-FISH procedure, no signal was found in all examined patients. Our data suggest that PBMCs infected with HCV can serve as a virus reservoir. HCV-RNA serum negative patients who have HCV-RNA in their leukocytes after completion of anti-viral therapy would be at great risk of hepatitis C recurrence. These HCV-RNA serum negative but PBMCs positive patients would be a potential source of HCV spread.