A critical point during mammalian pregnancy is the implantation of the blastocyst when the embryo attaches to the wall of the uterus. The autonomously developing preimplantation embryo then becomes dependent on the maternal environment for its continued development. Little is known about the regulation of implantation, except that a complex interaction between peptide and steroid hormones synchronizes the preparation of the uterus for implantation with the development of the embryo. Whether the implantation event is under maternal or embryonic control is also unclear (reviewed in refs 1, 2). We have previously shown that a cytokine, leukaemia inhibitory factor (LIF), is expressed in the uterine endometrial glands specifically on the fourth day of pregnancy. This burst of expression is under maternal control and always precedes implantation of the blastocyst. Here we report that transient expression of LIF in mice is essential for implantation. Females lacking a functional LIF gene are fertile, but their blastocysts fail to implant and do not develop. The blastocysts, however, are viable and, when transferred to wild-type pseudopregnant recipients, they can implant and develop to term.