OCT-4 (also known as POU5F1) is a key regulator of self-renewal in embryonic stem cells. Regarding the new cancer stem cell concept, the expression of such genes is potentially correlated with tumorigenesis and can affect some aspects of tumor behavior, such as tumor recurrence or resistance to therapies. Although OCT-4 has been introduced as a molecular marker for germ cell tumors, little is known about its expression in somatic cancers. Here, we have investigated the potential expression of OCT-4 in bladder cancer. We used semiquantitative RT-PCR to examine the expression of OCT-4 in 32 tumors, 13 apparently nontumor tissues taken from the margin of tumors and 9 normal urothelial tissues. The expression of OCT-4 at protein level was further determined by Western blotting and immunohistochemical (IHC) analysis. OCT-4 expression was detected in almost all examined tumors (31/32), but at much lower level (p < 0.001) in some nonneoplastic samples (6/22). A significantly strong correlation of 0.6 has been observed between OCT-4 expression and the presence of tumors (p < 0.001). Western blot analysis further confirmed the expression of OCT-4 in tumor biopsies. According to IHC results, OCT-4 is primarily localized in the nuclei of tumor cells, with no or low immunoreactivity in nontumor cells. Our study demonstrated, for the first time, the expression of OCT-4 in bladder cancer and a further clue to the involvement of embryonic genes in carcinogenesis. © 2007 Wiley-Liss, Inc.