Leukemia inhibitory factor (LIF) is essential for implantation of the embryo in the endometrium. It is not clear whether the blastocyst requires expression of LIF for implantation into tissues other than endometrium. Immunohistochemical localization of LIF was performed in the fallopian tube of 20 women with ectopic pregnancies, 7 women with normal pregnancies and 20 healthy non-pregnant women. Fallopian tubes were evaluated from specimens taken during tubal ligation in normal pregnancies and non-pregnant fertile women or at operation for tubal surgery in ectopic pregnancies. Biopsies were assayed by immunohistochemistry. Semi-quantitative immunohistochemical reaction scores (IRS) were used for immunohistochemical analyses. Immunolabeling of LIF was detected in the surface epithelium and stroma of fallopian tubes in all subjects. IRS score in the epithelium and stroma of non-pregnant women and women with intrauterine pregnancy were similar (p>0.05). However, women with ectopic pregnancy had significantly increased labeling of LIF compared to others (p<0.05). Immunohistochemical labeling of LIF in the fallopian tube was found to be increased in ectopic pregnancies compared to non-pregnant and healthy pregnant controls. This may indicate a role of LIF in the ectopic implantation of embryos.