Insulin-like growth factor-binding protein-1 (IGFBP1) is a major secretory product of the decidualized endometrium. In the present study, we investigated the role of two transcription factors, progesterone receptor (PGR) and a member of the forkhead box class O family of transcription factors (FOXO1A), in the regulation of the IGFBP1 gene in endometrial cells. Human endometrial fibroblasts (HuF) expressed FOXO1A, progesterone receptor A (PGRA), and progesterone receptor B (PGRB) proteins, whereas the endometrial adenocarcinoma cell line, HEC-1B cells, expressed only FOXO1A and no detectable PGR proteins. When FOXO1A expression was silenced using small interference RNA, IGFBP1 expression decreased in both HuF and HEC-1B cells. Using the chromatin immunoprecipitation technique, we demonstrated that liganded PGR was recruited to the IGFBP1 promoter region (-358 to -49). In addition, immunoprecipitation of HuF nuclear proteins with a PGR antibody followed by immunoblotting with anti-FOXO1A revealed that these two proteins interact in these cells. Reporter studies demonstrated that whereas liganded PGRA or PGRB increased a progesterone response element-linked reporter construct, pPRE/ GRE.E1b.Luc, coexpression of FOXO1A inhibited the PGRB response in HuF and synergistically increased PGRA and PGRB response in HEC-1B cells. Furthermore, in HEC-1B cells, FOXO1A increased IGFBP1 promoter activity, and coexpression of PGRA or PGRB further increased the promoter activity in a cooperative manner. In HuF, the response to FOXO1A and PGR was not additive; in fact, it was lower than the sum of the individual responses. Thus, FOXO1A and PGR associate with one another, and each influences the transactivating potential of the other. The cell type-dependent responses strongly implicate the involvement of other cofactors.