PURPOSE: In our previous studies we showed that upregulating claudin-6 (CLDN6) expression may contribute to preventing breast cancer, and that 17beta-estradiol induces a concentration- and time-related effect on CLDN6 mRNA and protein expression in MCF-7 cells. However, the mechanisms of 17beta-estradiol regulation of CLDN6 are still unclear. We determined the role of estrogen receptors in the regulation of CLDN6 expression in human breast cancer tissues and a cell line. METHODS: CLDN6, estrogen receptor alpha (ERalpha) and estrogen receptor beta (ERbeta) expression in breast cancer tissues were examined using immunohistochemistry. The human breast cancer cell line, MCF-7, which expresses ERalpha but not ERbeta was used. CLDN6 and ERalpha expression were measured by reverse transcriptase-PCR, Western blotting and immunofluorescent staining. Treatments with propyl pyrazole triol (PPT) and ICI 182, 780 (ICI) were performed. RESULTS: The results revealed that CLDN6 expression was related to ERalpha in breast cancer tissues (p=0.033). PPT, an ERalpha-selective ligand, upregulated CLDN6 expression at 10-5 mol/L after 24 hours. The effect of PPT on regulating CLDN6 expression in MCF-7 cells was blocked by ICI. CONCLUSION: These findings suggest that Eralpha reulates CLDN6 expression in breast cancer tissues and that 17beta-estradiol induces CLDN6 expression through an ERalpha pathway in MCF-7 cells.