Abstract Rhabdomyosarcoma (RMS) is a pediatric tumor believed to originate from perturbations of the normal development of the muscle lineage. The Polycomb Group (PcG) protein Ezh2, which was originally implicated in inhibiting homeobox gene expression during embryonic development, was recently shown to act as a negative regulator of muscle differentiation. Moreover, Ezh2 is a histone methyltransferase that is aberrantly overexpressed in most human cancers. Comparing the level of the expression of Ezh2 in RMS cell lines, a small group of tumor samples with normal skeletal muscle myoblasts (SkMC), and normal muscle tissues, we have previously observed that Ezh2 is highly upregulated in rhabdomyosarcoma tumors (Ciarapica et al., 2009). The aim of this study was to characterize the relevance of Ezh2 expression in RMS. Ezh2 expression was analyzed by immunohistochemistry in a cohort of 32 RMS primary tumors and correlated with the expression of the cell proliferation marker, Ki67, and the angiogenic marker, CD31. Results of these expression studies were then correlated with the clinicopathological features of the tumor samples. No significant difference in the clinicopathological variables was observed between the two major histophatological groups of RMS, the most aggressive alveolar RMS (ARMS) and the embryonal RMS (ERMS). Ezh2 expression was not correlated with the clinical parameters of the cohorts, but strikingly, a significant positive correlation was observed between Ezh2 and Ki67 expression in the whole RMS sample (r0 = 0,560; p = 0,013). The number of the large vessels was correlated with the total number of vessels (r0= 0,730; p<0,0001) and with the expression of Ki67 (r0= 0,421; p=0,02). In ARMS samples, the number of large vessels was significantly correlated with the expression of Ezh2 (r0= 0,559; p=0,047), the total number of vessels (r0= 0,781; p=0,02), and the expression of Ki67 (r0= 0,738; p=0,004). This study confirmed our previous observation regarding the aberrantly high expression of Ezh2 in rhabdomyosarcoma tumors on a larger cohort of samples. Moreover, the present data established a positive correlation between Ezh2 expression and the proliferative and angiogenic potential of rhabdomyosarcoma neoplasia. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3417.