A large, transient population of natural killer (NK) cells appears in the murine uterine mesometrial triangle during pregnancy. Depletion of uterine (u) NK cells, recently achieved using gene-ablated and transgenic mice, results in pathology. Pregnancies from matings of homozygous NK and T cell-deficient tg epsilon 26 mice have <1% of normal uNK cell frequency, no development of an implantation site-associated metrial gland, and an edematous decidua with vascular pathology that includes abnormally high vessel walls/lumens ratios. Fetal loss of 64% occurs midgestation and placentae are small. None of these features are seen in pregnant T cell-deficient mice. To confirm the role of the NK cell deficiency in these reproductive deficits, transplantation of tg epsilon 26 females was undertaken using bone marrow from B and T cell-deficient scid/scid donors. Engrafted pregnant females have restoration of the uNK cell population, induced metrial gland differentiation, reduced anomalies in the decidua and decidual blood vessels, increased placental sizes, and restoration of fetal viability at all gestational days studied (days 10, 12, and 14). Thus, uNK cells appear to have critical functions in pregnancy that promote decidual health, the appropriate vascularization of implantation sites, and placental size.