Adoptive T cell therapy (ACT) is a promising immunotherapeutic approach to fight cancer by transferring cytotoxic T lymphocytes (CTL), which specifically target and eradicate tumour cells. However, one major limitation of this therapy is the ability of tumours to interfere with the CTL through immune escape mechanisms, which may lead to poor CTL persistence and effector function. In a mouse model of B-cell lymphoma and ACT, we investigate the mechanisms underlying this failure. We found that tumour-antigen-specific CTL, upon transfer into a mouse with low tumour burden, successfully eradicate lymphoma cells, while upon encounter with a large tumour burden most of the CTL fail to survive and those that survive lose their effector functions. CTL death and loss of effector function are not induced by long-term persistence of antigen but were observed as early as 24-48 h after the adoptive transfer. Furthermore, CTL survival as well as killing ability and cytokine production were found to be dependent on the number of antigen expressing tumour cells rather than the total tumour burden. Our results describe a rapid deletion and inactivation mechanism of adoptively transferred CTL induced by encounter with a high number of antigen-presenting target cells.