Although T cells play important roles in the pathophysiology of ischemic stroke, the dynamics of T cells remains unclear. In cancer, polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) contribute to the maintenance of the tumor microenvironment by suppressing T cells. However, the presence of these cells has never been examined in ischemic brain. Therefore, we examined the temporal and spatial profiles of PMN-MDSCs, which are defined as the CD11b+Ly6ClowLy6G+ cells with higher expression levels of Nox2 and C/EBP Homologous Protein (CHOP) mRNA than normal neutrophil. Fluorescence-activated cell sorter (FACS) analysis showed that the count of CD11b+Ly6ClowLy6G+ cells was increased in the ischemic hemisphere and bone marrow at 72 hours, as well as in the spleen 24 hours after transient middle cerebral artery occlusion in mice. In contrast, the contralateral hemisphere, normal bone marrow, and normal spleen contained few CD11b+Ly6ClowLy6G+ cells. Real-time reverse transcription polymerase chain reaction revealed that CD11b+Ly6ClowLy6G+ cells sorted from brain and spleen 72 hours after ischemia had greater expression of Nox2 and CHOP mRNA than neutrophils in bone marrow, suggesting that these cells constitute PMN-MDSCs. Immunohistochemistry showed that CD11b+Ly6G+ cells were located in the ischemic core and border zone, indicating that PMN-MDSCs might be endemic to these regions. Although neutrophils are believed to invade infarct regions 48-72 hours after ischemia, the present study suggested that some of these cells are in fact PMN-MDSCs. Further studies on the function of PMN-MDSCs might unveil the unknown mechanisms of T cell activation and recruitment in ischemic stroke.