Background/objective: Studies of the decidual leukocyte populations in women suffering from spontaneous early pregnancy loss may provide insight into immunpathological aspects and even cast light on the etiology of recurrent spontaneous abortion.Methods: In order to clarify the immunological role of endometrial leukocytes in repetitive abortion of unknown etiology, a comparative analysis of phenotypes of leukocyte sub-populations was performed on first-trimester decidual tissues obtained from thirty patients with recurrent spontaneous abortion and thirty samples at therapeutic abortion. Paraffin-embedded sections of these samples were labeled with a panel of monoclonal antibodies against leukocyte markers, CD68, CD3, CD45RA, CD57 using streptavidin-conjugated peroxidase technique. The numbers of positive cells for each monoclonal antibody for all control and pathological cases were counted microscopically at ´400 magnification.Results: Many macrophages were observed in pathological decidual tissues compared with normal early pregnancy decidua, however the difference was not significant (p=0.36). There was no significant difference in the number of T cells and B cells despite a slight decrease in recurrent spontaneous aborters (p=0.36; p=0.15, respectively). Scantly CD57-positive classical NK cells were in normal pregnancy decidua. Although the number of classical CD57-positive natural killer cells varies in pathological cases compared with normal pregnancy cases, the difference between the two groups reached significant level (p=0.003).Conclusion: These findings suggest that classical NK cells possibly play a role together with other activated decidual cell populations in the control of trophoblast growth and placental development.