ARTICLE ABSTRACTCancer stem–like cells (CSC) may be critical to maintain the malignant behavior of solid and hematopoietic cancers. Recently, patients with endometrial cancer whose tumors expressed high levels of aldehyde dehydrogenase (ALDH), a detoxifying enzyme characteristic of many progenitor and stem cells, exhibited a relative reduction in survival compared with patients with low levels of ALDH. Given evidence of its role as a CSC marker, we hypothesized that high level of ALDH activity (ALDHhi) in a tumor might positively correlate with the presence of stem- and progenitor-like tumor cells in this disease setting. In support of this hypothesis, ALDH could be used to enrich for CSC in endometrial cancer cell lines and primary tumors, as illustrated by the increased tumor-initiating capacity of ALDHhi cells in immunodeficient mice. ALDHhi cells also exhibited greater clonogenic and organoid-forming capacity compared with ALDHlo cells. Notably, the number of ALDHhi cells in tumor cell lines and primary tumors inversely correlated with differentiation grade. Expression analysis revealed upregulation of IL6 receptor subunits and signal transducers CD126 and GP130 in ALDHhi endometrial cancer cells. Accordingly, targeted inhibition of the IL6 receptor and its downstream effectors JAK1 and STAT3 dramatically reduced tumor cell growth. Overall, our results provide a preclinical rationale to target IL6 or its effector functions as a novel therapeutic option in endometrial cancer. Cancer Res; 75(17); 3608–22. ©2015 AACR.