To investigate the cytokine expression by peripheral blood natural killer (NK) cells of women with recurrent spontaneous abortion (SAB) or implantation failures. Prospective cohort study. University clinic. Twenty-five women with recurrent SAB, 20 women with implantation failures, and 15 healthy controls. None. Cytokine expression (interferon-gamma, tumor necrosis factor [TNF]-alpha, interleukin [IL]-4, IL-5, IL-10, IL-13, granulocyte-macrophage colony-stimulating factor [GM-CSF]) in NK cells and their subsets (CD56(dim) and CD56(bright)). Proportion (percentage) of CD56(bright)/interferon-gamma(+)/TNF-alpha(+) cells was significantly higher in women with recurrent SAB and implantation failures as compared with that of healthy controls. Proportion of CD56(bright)/IL-4(+)/IL-10(+) cells was very low (<2%) in all groups but was significantly lower in women with recurrent SAB than that of controls. The TNF-alpha/GM-CSF expressing CD56(bright) cell ratio was significantly higher in women with recurrent SAB and implantation failures than in controls. Natural killer-1 shift in peripheral blood NK cells was identified in nonpregnant women with recurrent SAB and implantation failures. Tumor necrosis factor-alpha/GM-CSF expressing CD56(bright) cell ratio can be applicable for the diagnosis of recurrent SAB or implantation failures. Further studies are needed as to whether cytokine expression of NK cells during pregnancy can affect pregnancy outcome.