Problem-Natural Killer (NK) cell numbers and cytotoxicity are suppressed during pregnancy. Little is known about postpartum NK number and function.Method of study-Postpartum women (n = 39) were studied at one week and then monthly over the first six postpartum months. The standard natural killer cell cytotoxicity assay (NKCA) was performed. This is a Cr51 release assay from K562 cells cultured with peripheral blood mononuclear cells (PBMCs).Results-Data indicate suppression of NK cytotoxicity in postpartum women. Cytotoxicity at each effector:target (E:T) ratio showed a drop from 1 week postpartum, reaching a nadir at around 2 months, and a trend towards recovery of cytotoxicity from 3 to 6 months. Lytic units (LUs) from pre-incubated cells from postpartum women were lower than age-matched, non-pregnant, nonpostpartum controls through the fifth postpartum month.Conclusion-These data suggest that the postpartum period, like pregnancy, is characterized by decreased NK cytotoxicity activity. This suppressed NK cytotoxic effect may result as a response to interaction with tolerized fetal microchimeric cells accumulated during pregnancy in maternal blood and tissues.