Abstract Purpose: To investigate whether radium-223 increases peripheral immune responses to sipuleucel-T in men with bone-predominant, minimally symptomatic metastatic castration-resistant prostate cancer (mCRPC). Patients and Methods: A total of 32 patients were randomized 1:1 in this open-label, phase II multicenter trial. Patients in the control arm received three sipuleucel-T treatments, 2 weeks apart. Those in the combination arm received six doses of radium-223 monthly, with sipuleucel-T intercalated between the second and fourth doses of radium-223. The primary endpoint was a comparison of peripheral antigen PA2024-specific T-cell responses (measured by proliferation index). Secondary endpoints were progression-free survival (PFS), overall survival (OS), and PSA responses. Results: We enrolled 32 patients, followed for a median of 1.6 years. Six weeks after the first sipuleucel-T dose, participants in the control arm had a 3.2-fold greater change in PA2024-specific T-cell responses compared with those who received combination treatment (P = 0.036). Patients in the combination arm were more likely to have a >50% PSA decline [5 (31%) vs. 0 patients; P = 0.04], and also demonstrated longer PFS [39 vs. 12 weeks; HR, 0.32; 95% confidence interval (CI), 0.14–0.76] and OS (not reached vs. 2.6 years; HR, 0.32; 95% CI, 0.08–1.23). Conclusions: Our data raise the possibility of greater clinical activity with the combination of sipuleucel-T and radium-223 in men with asymptomatic bone mCRPC, despite the paradoxically lower immune responses observed. Additional study to confirm these findings in a larger trial is warranted.