Purpose: Acute myelogenous leukemia (AML) blasts are able to differentiate into leukemia-derived dendritic cells (AML-DC), thereby enabling efficient presentation of known and unknown leukemic antigens. Advances in culture techniques and AML-DC characterization justify clinical application. However, additional measures are likely needed to potentiate vaccines and overcome the intrinsic tolerant state of the patients' immune system. Engagement of the costimulatory molecule 4-1BB can break immunologic tolerance and increase CTL responses. In this study, we examined the role of the 4-1BB ligand (4-1BBL) on T-cell responses induced by AML-DC. Experimental Design: In allogeneic and autologous cocultures of T cells and AML-DC, the effect of the addition of 4-1BBL on T-cell proliferation, T-cell subpopulations, and T-cell function was determined. Results: Addition of 4-1BBL to cocultures of AML-DC and T cells induced a preferential increase in the proliferation of CD8+ T cells. Increased differentiation into effector and central memory populations was observed in both CD4+ and CD8+ T cells in the presence of 4-1BBL. AML-DC induce a T helper 1 response, characterized by high IFN-γ production, which is significantly increased by targeting 4-1BB. T cells primed in the presence of 4-1BBL show specificity for the leukemia-associated antigen Wilms' tumor 1, whereas cytotoxicity assays with leukemic blast targets showed the cytolytic potential of T cells primed in the presence of 4-1BBL. Conclusion: We conclude that 4-1BBL is an effective adjuvant to enhance T-cell responses elicited by AML-DC.