Abstract Anti–4-1BB and cisplatin showed synergistic anticancer effects in the CT-26 colon carcinoma model, producing complete regression in >60% of mice with either preventive or therapeutic treatment. The tumor-free mice formed long-lasting CD8+ T cell–dependent tumor-specific memory. Anti–4-1BB induced rapid repopulation of T and B cells from cisplatin-mediated lymphopenia and differentiation and expansion of IFN-γ+CD11c+CD8+ T cells. Cisplatin facilitated expansion of naïve, effector, and memory CD8+ T cells; combination therapy produced almost twice as many lymphoid cells as anti–4-1BB alone. Cisplatin increased 4-1BB on antigen-primed T cells and induced 4-1BB de novo on kidney tubular epithelium. Cross-linking of 4-1BB protected the T cells and kidney epithelium from cisplatin-mediated apoptosis by increasing expression of antiapoptotic molecules. Thus, cisplatin-induced 4-1BB provided a mechanism for amelioration of the lymphopenia and nephrotoxicity inherent in cisplatin treatment. We concluded that chemoimmunotherapy with anti–4-1BB and cisplatin is synergistic in tumor killing and prevention of organ-specific toxicity. [Cancer Res 2008;68(18):7264–9]