Sarcomas are rare but aggressive malignant tumors associated with high mortality, for which the efficacy of standard therapies remains limited. In order to develop immunotherapeutic approaches for the treatment of sarcoma, we studied the relevance of cancer/testis antigens (CTAs), a group of antigens whose expression is developmentally regulated and that is specifically found in some tumor types, as sarcoma vaccine targets. CTA expression was assessed by PCR and/or immunohistochemistry (IHC) in sarcoma tumor samples that included different histological subtypes and sarcoma cell lines. Expression of HLA class I was assessed by IHC in tumor samples and by FACS analysis in cell lines. More than 70% of the tumor samples expressed at least one CTA. The majority of tumors and cell lines expressed normal levels of HLA class I. HLA class I expression in cell lines was enhanced upon treatment with IFN-gamma. CTA expression was enhanced or induced by treatment with the demethylating agent 5-aza-2'-deoxycytidine, resulting in recognition by specific CTLs. Interestingly, a spontaneous humoral and CD8+ T cellular response to the CTA NY-ESO-1 was detected in a synovial sarcoma patient. Together, these findings strongly support the implementation of CTA-based immunotherapy of sarcoma as a means to improve the efficacy of the standard therapy.