Surgical androgen deprivation therapy (ADT) has activating effects on the immune system. However, whether medical ADT (with androgen receptor (AR) antagonists) has similar effects is not clear. Pu et al. report the unexpected finding that AR antagonists suppress immune responses by inhibiting T cell activation. Furthermore, surgical ADT synergized with immunotherapy in mouse models of prostate cancer, whereas medical ADT suppressed immunotherapy effects. These findings might improve prostate cancer immunotherapy.