Abstract Follicular lymphomas (FLs) rarely induce clinically significant T-cell–mediated responses. We showed that freshly isolated tumor infiltrating T cells (T-TILs) lack tumor-specific cytotoxicity. Stimulation of these T cells with FL cells in the presence of interleukin-2 (IL-2) and/or costimulation via CD28 does not lead to T-cell activation and expansion. In contrast, when stimulated with FL cells preactivated via CD40, autologous T-TILs can be expanded by the addition of exogenous IL-2. These T cells can be further expanded in vitro by the addition of exogenous IL-4, IL-7, or interferon-γ, but not IL-12. Once activated, these T cells showed FL-directed cytotoxicity in four of five patients tested. We concluded that autologous cytotoxic anti-FL–specific T cells exist, but can only be detected in vitro under optimized conditions for T-cell stimulation and expansion. This suggests that their frequency in vivo is either very low or that the microenvironment does not provide the necessary signals to activate these T cells. This model system allows dissection of the requisite conditions for activation and expansion of lymphoma-directed cytotoxicity and may permit expansion of previously activated cytotoxic T cells for adoptive transfer.