Tumor-infiltrating cytotoxic T lymphocytes (TILs), including CD8 TILs, have been associated with favorable clinical outcomes in multiple tumor types. Tumor-infiltrating CD8 T cells and major histocompatibility complex (MHC) class I expression in urothelial carcinoma (UC) have not been previously reported. Most immune responses are mediated by local cytotoxic lymphocytes (CD8 T cells), which can eradicate tumor cells by recognizing tumor-associated antigens presented by MHC class I molecules. Here we analyzed the presence of intratumoral CD8 T cells, the expression of MHC class I antigen, and the expression of the NY-ESO-1 tumor antigen in UC samples and correlated our findings with clinical outcome. Immunohistochemical staining for intratumoral CD8 T cells in tissue samples from 69 patients with UC showed that patients with advanced UC (pT2, pT3, or pT4) and higher numbers of CD8 TILs within the tumor (> or =8) had better disease-free survival (P < 0.001) and overall survival (P = 0.018) than did patients with similar-staged UC and fewer intratumoral CD8 TILs. We conclude that the extent of intratumoral CD8 TILs is an important prognostic indicator in advanced UC.