The objective of this study is to examine the localization and expression of FOXP1 in human endometrium during the menstrual cycle and in endometriotic lesions and endometrial adenocarcinoma. FOXP1 protein expression was analyzed by immunohistochemistry and Western blot. FOXP1 expression was significantly different between glandular epithelial and stromal nuclei and cytoplasm in both endometrial functionalis and basalis. FOXP1 immunostaining was significantly reduced in the early secretory stage in comparison to the mid proliferative stage in the functionalis and the early proliferative stage in the basalis. FOXP1 expression was found in endometriotic lesions but not in endometrial adenocarcinoma. Multiple protein bands of FOXP1 were identified, and their presence varied considerably among patients. Protein expression levels were significantly higher in the mid and late secretory stages in comparison to early proliferative and early secretory stages. FOXP1 protein is present in human endometrium with evidence of cycle stage-dependent changes in expression.