PROBLEM: The contribution of myeloid-derived suppressor cells (MDSC) in patients suffering from early or recurrent miscarriage is unknown. MDSC are implicated in modulation of T-cell response in healthy pregnancies; however, the role of MDSC in patients suffering from miscarriage has not been studied. We hypothesized that MDSC play major role in inducing maternal-fetal tolerance and this tolerance is compromised in patients suffering from miscarriage. METHOD OF STUDY: MDSC level was assessed by flow cytometry and immunostaining in blood and endometrial decidua, respectively. Activation of T cells was determined by MTT proliferation and IL-2 ELISA assays. RESULTS AND CONCLUSION: The miscarriage patients harbor reduced level of functionally suppressive MDSC in blood and endometrium as compared to healthy control women with successful pregnancies. These results suggest MDSC regulate maternal tolerance in healthy pregnancies and that drug inducing MDSC could have therapeutic implication in the miscarriage patients.