The transformation of endometrium into decidua is an essential feature of normal implantation and pregnancy. There is a close association with an unusual leukocyte population, uterine natural killer (NK) cells, and onset of decidualization. These uterine NK cells are seen in close contact with stromal cells ultrastructurally and are also seen encircling vessels and glands. The possibility that uterine NK cells in the late secretory phase and in early decidua may be important in initiating and maintaining decidualization is raised. In contrast, the death of uterine NK cells could be an early event in the onset of endometrial breakdown at menstruation. The period between implantation and menstruation (7-14 days after luteinizing hormone surge) is the time when implantation is known to be particularly vulnerable. In this review, the possibility that uterine leukocytes might influence the critical decision that the mid- to late secretory endometrium must make either to decidualize or to undergo menstruation is explored.