Purpose: Effective treatments for advanced endometrial cancer are lacking. Novel therapies that target specific pathways hold promise for better treatment outcomes with less toxicity. Mutation activation of the FGFR2/RAS/ERK pathway is important in endometrial tumorigenesis. RPS6KA6 (RSK4) is a putative tumor suppressor gene and is a target of the ERK signaling pathway. We explored the role of RSK4 in endometrial cancer. Experimental Design: We showed that RSK4 is expressed in normal endometrial tissue and is absent or much reduced in endometrial cancer. On the basis of previous reports on methylation in other cancers, we hypothesized that the absence of RSK4 transcript is associated with epigenetic silencing rather than mutation. We determined the methylation and expression status of RSK4 in primary endometrial cancers and cell lines and the effects of treatment with a demethylating agent. The relationship between RSK4 methylation and clinicopathologic features was assessed. Results: RSK4 is frequently hypermethylated in endometrial cancer cells lines and in primary endometrial cancer compared with normal endometrial tissue. RSK4 methylation was significantly associated with tumor grade, with highergrade tumors having lower levels of methylation(P = 0.03).RSK4 methylation levels were not associated with other clinical variables. We did find that RSK4 methylation was significantly correlated with expression in primary endometrial tumors and in cell lines. Reactivation of RSK4 by 5-azacytidine was successfully performed showing 8-to more than 1,200-fold increases in transcript levels. Conclusion: RSK4 appears to be epigenetically silenced in endometrial cancer as evidenced by hypermethylation. Its role as a suppressor in endometrial cancer, however, remains uncertain.