Myeloid-derived suppressor cells (MDSC) are one of the major cell populations responsible for regulating immune responses. These cells have been reported to accumulate in the blood, lymph nodes, and tumor sites in most patients during tumor progression and in chronic infection. They are also reported to potently suppress T-cell functions. We studied MDSC in the peripheral blood mononuclear cells(PBMC)by flow cytometry using blood samples from 29 patients with breast cancer, and from 11 healthy donors. The cell level was significantly high for patients compared to the 11 healthy donors (5. 68±6. 09% vs. 0. 91±0. 54%). MDSC was significantly higher in all of the breast cancer patients (5. 68±6. 09%), preoperative patients (5. 79±4. 92%) and recurrent disease patients (5. 59±7. 28%), compared to healthy donors, but not for postoperative patients (1. 50±0. 95%). Thus, MDSC was elevated in patients with breast cancer, but decreased to the range of healthy individuals after the removal of the tumor mass. However, MDSC increased again with recurrence. We also report that in 2 cases, MDSC in the peripheral blood and pleural effusion of patients with metastatic breast cancer decreased after chemotherapy with gemcitabine.