In prostate cancer, there is considerable evidence that tumors promote immune tolerance starting early in the disease. By suppressing tumors and activating immune system homeostatic mechanisms, chemotherapy may help overcome this tumor-induced immune tolerance. As such, chemotherapy may therefore support improved results from novel immune-modulating therapies. Prostate cancer is particularly suited for active immunotherapy because prostate tumor cells express a number of distinctive surface antigens. Sipuleucel-T, which has recently been approved in the United States, is an active immunotherapy that triggers T-cell responses against prostate cancer. An exploratory analysis of phase III trial participants found a substantial survival benefit to receiving docetaxel some months after sipuleucel-T. However, VITAL-2, a phase III trial investigating a prostate cancer therapeutic vaccine plus concurrent docetaxel versus standard docetaxel therapy in advanced prostate cancer, observed lower overall survival with the vaccine regimen. This trial highlights major unresolved questions concerning the optimum choice, dosing, and timing of chemotherapy relative to active immunotherapy. Patient characteristics, prostate cancer disease stage, and treatment history also may influence the response to combined therapy. Advances in biomarker validation and trial design are needed to efficiently investigate these issues.