Myeloid-derived suppressor cells (MDSCs) are heterogeneous, immature, myeloid progenitor cells, which suppress immune responses against tumors. CD14(+)HLA-DR(-/low) monocytic MDSCs (M-MDSC) are increased in patients suffering from multiple myeloma (MM). However, the frequency and function of M-MDSCs with the relationship between the tumor development and outcome of therapy in MM remain unclear. In this study, we analyzed the changes in M-MDSCs in newly diagnosed, relapsed and remission MM patients. In addition, we also assessed the response of M-MDSCs in MM patients treated with a bortezomib-based therapy as well as the impact of bortezomib on the modulation of M-MDSCs in vitro. The levels of M-MDSCs in newly diagnosed and relapsed MM patients were significantly increased compared with those in remission MM patients and healthy donors. Moreover, the levels of M-MDSCs were shown to correlate with tumor progression. The decrease in M-MDSCs after proteasome inhibitory therapy suggested that M-MDSCs could be considered as an indicator for the efficacy of therapy. Finally, we found the plasma from newly diagnosed MM patients, and MM cells were able to induce the accumulation of M-MDSCs in vitro. These results indicated that M-MDSCs could be considered as a prognostic predictor and an important cell type contributing to immune suppressive microenvironment in MM patients. Treatments targeting for M-MDSCs may improve therapeutic outcomes for MM patients.