Myeloid-derived suppressor cells (MDSCs) have been shown to inhibit T-cell responses in many diseases, but, in hepatitis C virus (HCV) infected patients, MDSCs are still poorly studied. In this assay, we investigated the phenotype and frequency of two new populations of MDSCs denoted as monocytic and granulocytic MDSCs (M-MDSCs and G-MDSCs) in HCV infected patients and analyzed their clinical significance in these patients respectively. We found that the frequency of CD14(+)HLA-DR-/low cells (M-MDSCs) from HCV infected patients (mean +/- SE, 3.134% +/- 0.340%) was significantly increased when compared to healthy controls (mean +/- SE, 1.764% +/- 0.461%) (Z = -2.438, P = 0.015), while there was no statistical difference between the frequency of HLA-DR(-/low)CD33(+)CD11b(+)CD15(+) (G-MDSCs) of HCV infected patients and healthy donors (0.201% +/- 0.038% versus 0.096% +/- 0.026%, P > 0.05), which suggested that HCV infection could cause the proliferation of M-MDSCs instead of G-MDSCs. Besides, we found that the frequency of M-MDSCs in HCV infected patients had certain relevance with age (r = 0.358, P = 0.003); patients older than 40 years old group (mean +/- SE, 3.673% +/- 0.456%) had a significantly higher frequency of M-MDSCs than that of age less than 40 years old group (mean +/- SE, 2.363% +/- 0.482%) (Z = -2.685, P = 0.007). The frequency of M-MDSCs, however, had no correlation with HCV RNA loads, aspartate aminotransferase (AST), alanine aminotransferase (ALT), and the level of liver inflammation degree.