Background: Testosterone suppression (TS) is the backbone of treatment for mHSPC. OS is improved by the addition of early docetaxel (DOC) or abiraterone to TS. ENZAMET assessed the effects of enzalutamide (ENZA), a potent androgen receptor (AR) inhibitor, versus a nonsteroidal anti-androgen (NSAA: bicalutamide, nilutamide, or flutamide) in addition to SOC (TS with or without DOC) in mHSPC. Methods: Men with mHSPC were randomly assigned 1:1 to receive TS plus either ENZA or NSAA. Randomization was stratified by: volume of disease (high vs low, according to CHAARTED); planned early DOC; planned anti-resorptive therapy, comorbidity score (ACE-27), and study site. The primary endpoint was overall survival. Accrual of 1100 men provided 80% power to detect a 25% reduction in the hazard of death (HR 0.75) with up to 4 interim analyses (IA), the first planned to occur after 235 deaths (50% of total information with a critical p-value threshold <0.0031 by the Lan-DeMets alpha-spending approach with O’Brien-Fleming type shape). Subgroup analyses to assess possible modulation of the treatment effect were specified a priori and included planned early docetaxel (yes vs no) and volume of disease (high vs low). Results: We randomly assigned 1125 patients from 31MAR14 to 24MAR17. The treatment groups were well balanced for all important baseline factors. Criteria for early reporting were met at the first IA (28FEB2019) after a median follow-up of 33 months. Overall survival was prolonged by ENZA (see below). At 3 years, 36% NSAA vs 64% ENZA were still on their assigned study treatment. Serious adverse events (regardless of attribution) within 30 days of study treatment occurred in 42% ENZA vs 34% NSAA, commensurate with the different durations of study treatment. Conclusions: ENZA significantly improved OS when added to SOC in mHSPC. The benefits appeared lower in those planned to receive early DOC. Results of analyses with updated follow-up triggered by this IA will be presented. Clinical trial information: NCT02446405