Although hepatitis C is mainly hepatotropic, some studies suggest that hepatitis C virus (HCV) infects peripheral blood mononuclear cells (PBMC), using them as a reservoir, which might contribute to the development of resistance to treatment. Fifty-four hepatitis-C patients, who had been submitted to treatment, were selected. Blood samples were collected on the same day for the detection of HCV RNA in serum and PBMC by PCR, using the Amplicor HCV 2.0 assay (Roche Diagnostics). HCV genotyping was performed using the INNO-LiPA HCV kit (Versant, Bayer Diagnostics). HCV RNA was detected in both serum and PBMC in 35 (64%) patients and no RNA in 16 (29.6%). Disagreement between the serum and PBMC results was observed for three patients (5.6%), with HCV RNA being detected in PBMC but not in serum. Four months later, new serum and PBMC samples were collected from one of these patients and HCV RNA was detected in both samples, showing that PBMC can reveal signs of a lack of response to treatment. We conclude that the absence of HCV in the serum of patients with chronic hepatitis C by the end of treatment does not mean that there is no circulating virus. HCV in mononuclear cells may be an indicator of the persisting infection.