Enzalutamide, a potent oral androgen receptor inhibitor, improves survival in men with metastatic castration-resistant prostate cancer (CRPC) before and after chemotherapy. Bicalutamide, a nonsteroidal antiandrogen, is widely used to treat men with nonmetastatic or metastatic CRPC. The efficacy and safety of these drugs were compared in this randomized, double-blind, phase II study of men with CRPC. A total of 396 men with nonmetastatic (n = 139) or metastatic (n = 257) CRPC were randomly assigned to enzalutamide 160 mg per day (n = 198) or bicalutamide 50 mg per day (n = 198). Androgen deprivation therapy was continued in both arms. The primary end point was progression-free survival (PFS). Enzalutamide reduced the risk of progression or death by 76% compared with bicalutamide (hazard ratio HR, 0.24; 95% CI, 0.18 to 0.32; P < .001). Median PFS was 19.4 months with enzalutamide versus 5.7 months with bicalutamide. Enzalutamide resulted in significant improvements in all key secondary end points: time to prostate-specific antigen progression (HR, 0.19; 95% CI, 0.14 to 0.26; P < .001); proportion of patients with a ≥ 50% prostate-specific antigen response (81% v 31%; P < .001); and radiographic PFS in metastatic patients (HR, 0.32; 95% CI, 0.21 to 0.50; P < .001). Beneficial effects with enzalutamide were observed in both nonmetastatic and metastatic subgroups. The observed adverse event profile was consistent with that from phase III enzalutamide trials. Enzalutamide significantly reduced risk of prostate cancer progression or death compared with bicalutamide in patients with nonmetastatic or metastatic CRPC.