The interaction between FasL on tumor cells and Fas on lymphocytes may represent a tumor immune escape mechanism. We explored FasL expression and function in human urinary bladder transitional cell carcinomas (TCCs). FasL expression was observed in situ in 45% of TCCs (n ‫؍‬ 45) and was absent in normal urothelium (n ‫؍‬ 20). A correlation existed between FasL expression and high tumor grade (0% in G1, 14% in G2, and 75% in G3; P < 0.0001) and stage (13% in superficial Ta-T1 versus 81% in invasive T2-T4; P < 0.0001). Urinary bladder cancer is the fifth leading cause of cancer death among males in Western countries.1 Transitional cell carcinoma (TCC) is the most common bladder cancer, which in ϳ80% of the cases occurs as superficial (confined to the urothelium, stage Ta; or the lamina propria, stage T1) and in ϳ20% as invasive (penetrating the muscle, stages T2-T4) tumor. The prognosis of TCC is directly related to the stage of the tumor. In superficial tumors a 5-year-survival rate of 80 to 90% of the patients is currently described, whereas in invasive tumors less than 40% of the patients reach a 5-year survival.