Background: PROSPER previously demonstrated a statistically significant and clinically meaningful improvement in metastasis-free survival (MFS) (hazard ratio [HR] 0.29; 95% CI 0.24-0.35; P< .001) in men with nmCRPC and rapidly rising prostate-specific antigen (PSA) who received ENZA. When first reported, OS was immature with only 165 of 596 (28%) prespecified deaths. Here we report results from the final OS analysis. Methods: Men with nmCRPC, PSA doubling time ≤ 10 mo, and PSA ≥ 2 ng/mL at screening continued androgen deprivation therapy (ADT) and were randomized 2:1 to ENZA 160 mg or PBO. OS treatment effect was assessed using a group sequential testing procedure with O’Brien-Fleming-type alpha spending function (P≤ .021 required for statistical significance). Medians were estimated using the Kaplan-Meier method; 95% CIs using a stratified Cox regression model. Results: As of Oct 15, 2019 (median follow-up ≈ 48 mo), there were 466 deaths (288 [30.9%] and 178 [38.0%] in the ENZA and PBO arms, respectively). ENZA significantly prolonged OS compared with PBO (HR 0.73; 95% CI 0.61-0.89; P = .0011). Median OS was 67.0 mo (95% CI 64.0-not reached) in the ENZA arm and 56.3 mo (95% CI 54.4-63.0) in the PBO arm. Subsequent antineoplastic therapies were initiated after treatment discontinuation by 310 (33%) men in the ENZA arm vs 303 (65%) in the PBO arm. Median duration of treatment was 33.9 mo vs 14.2 mo with ENZA vs PBO, respectively. Grade ≥ 3 adverse events (AEs) were reported by 48% of men in the ENZA arm vs 27% in the PBO arm (16% vs 6% were drug related, respectively). AEs with event rates per 100 patient-yr that were ≥ 2 points higher with ENZA vs PBO were falls (9 vs 4), fatigue (14 vs 12), and hypertension (7 vs 5). Conclusions: ENZA treatment resulted in a statistically significant 27% reduced risk of death compared with PBO, demonstrating that initiation of ENZA + ADT before the onset of detectable metastasis improves OS in men with CRPC and rapidly rising PSA. This OS benefit ensues despite crossover from the PBO arm to ENZA and higher rates of subsequent antineoplastic therapies in men from the PBO arm. Safety was consistent with previous clinical trials. This final OS analysis from PROSPER provides prospective validation of MFS as a potential surrogate endpoint for OS in nmCRPC and supports the continued use of ENZA + ADT as a standard of care in men with nmCRPC and rapidly rising PSA. Clinical trial information: NCT02003924.