Objectives: To localize indoleamine 2,3-dioxygenase (IDO) mRNA and protein and to undertake a functional study at the first trimester fetal-maternal interface in order to determine whether the distribution and function of IDO are related to recurrent spontaneous abortion (RSA). Methods: Women undergoing legal pregnancy termination and women with RSA participated in this prospective study. Immunohistochemistry and real-time reverse transcription-polymerase chain reaction were used to analyse levels of IDO protein and mRNA in placenta, decidua and HTR-8/SVneo cells. Culture medium collected from trophoblast villous explant or HTR-8/SVneo cell cultures was used to measure IDO activity in response to interferon (IFN)-g treatment. Results: A total of 40 healthy women and 26 women with RSA provided samples of placenta and decidua. For normal pregnancies, IDO protein and mRNA was identified in placental trophoblasts, invasive extravillous trophoblasts and decidual glandular epithelium. IFN-g significantly increased IDO activity in trophoblast villous explants and HTR-8/SVneo cells. Levels of IDO protein and mRNA in the placenta and decidua from normal pregnancies were significantly higher than in those from RSA. Conclusions: Decreased levels of IDO protein and mRNA in the placenta and decidua from RSA suggest an important role for IDO in the maintenance of normal pregnancy.