Background: This trial compared the efficacy of DE-EBRT and LDR-B for National Comprehensive Cancer Network (NCCN) high and intermediate-risk disease. Methods: A planned sample size of 400 patients were randomized to one of two treatment arms and stratified by risk group. Both arms received 12 months of androgen deprivation therapy (ADT) with luteinizing hormone releasing hormone (LHRH) agonist plus a non-steroidal anti-androgen for at least 1 month. After 8 months of neo-adjuvant ADT, both arms received whole pelvis EBRT (46Gy/23#). Patients assigned to DE-EBRT (standard arm) then received a conformal EBRT boost (32Gy/16#). Patients assigned to LDR-B (experimental arm) received an Iodine-125 LDR boost prescribed to a minimum peripheral dose of 115Gy. The primary endpoint was relapse free survival (RFS) defined by biochemical criteria using the nadir+2 ng/mL threshold. Time zero was the date of the first LHRH injection. Results: Between Dec 2002 and Sep 2011, 276 high-risk and 122 intermediate-risk patients were accrued at 6 cancer treatment centers. 200 men were assigned to DE-EBRT and 198 to LDR-B. The treatment arms were well balanced in terms of age and known prognostic factors. Median follow up (FU) is 6.5 years; 65 men have >9 years FU. There were 12 major protocol violations in each arm. By intent-to-treat analysis, the 3-, 5-, 7-, and 9-year Kaplan-Meier RFS estimates are 94% vs 94%, 77% vs 89%, 71% vs 86%, and 63% vs 83% for DE-EBRT and LDR-B respectively (hazard ratio = 0.473; 95% CI 0.292 – 0.765; P = 0.0022). Randomization (p<0.001), percent positive cores (p=0.005), initial PSA (p=0.006) and clinical T-stage (p=0.013) were predictive of RFS in a multivariable Cox model. The median PSA at latest FU for non-relapsing patients assigned to LDR-B is 0.02 vs 0.24 ng/mL for DE-EBRT. Conclusions: In a randomized trial, an Iodine-125 LDR boost was much more effective than an EBRT boost in rendering unfavorable-risk prostate cancer patients biochemically disease free. *ASCENDE-RT- Androgen Suppression Combined with Elective Nodal and Dose Escalated Radiation Therapy is an NCI registered trial (NCT00175396). Clinical trial information: NCT00175396