123 Background: The low α\β ratio of prostate cancer (PCa), 1.5-2, suggests high radiation-fraction sensitivity and predicts a therapeutic advantage of hypofractionated radiation treatment (HFRT). Most available data of moderate HFRT have focused on low, intermediate and/or mixed risk groups. We therefore conducted the first randomized trial of moderately HFRT in high-risk PCa patients and present the primary safety analysis of side effects at 2 years. Methods: We conducted a Canadian multi-centric phase III trial of conventional fractionated radiation therapy (CFRT) vs. intensity-modulated HFRT in men with high-risk PCa as per NCCN definition. From February 2012 to March 2015, 329 patients were randomized in a 1:1 ratio to receive either CFRT or HFRT. All patients received neo-adjuvant, concurrent and adjuvant androgen suppression, with a median duration of 24 months. CFRT consisted of 76 Gy in 2 Gy per fraction to the prostate where 46 Gy was delivered to the pelvic lymph nodes. HFRT consisted of concomitant dose escalation of 68 Gy in 2.72 Gy per fraction to the prostate and 45 Gy, in 1.8Gy per fraction to the pelvic lymph nodes. The primary endpoint was to compare the toxicities at 6 months and at 24 months using the CTCAE v.4. Results: Of the329 patients, 164 were randomized to HFRT and 165 to CFRT. The minimum, median and maximum follow-up were 24, 40 and 60 months respectively. At 24 months, 12 patients in the CFRT arm and 15 patients in the HFRT arm had grade 2 or worse gastrointestinal (GI)-related adverse events (HR:1.32 [0.62.2.83] 95% CI; P=NS). Similarly, 11 patients in the CFRT arm and 3 patients in HFRT arm had grade 2 or higher genitourinary (GU) toxicities (HR:0.26 [0.07-0.94] 95% CI; P=0.037). In the HFRT arm, there were 3 grade 3 GI and one grade 3 GU related toxicities. In the CFRT arm there were 3 grade 3 GU and no grade 3 GI related toxicities. There were no grade 4 toxicities in either arm. Conclusions: This is the first hypofractionated dose escalated radiotherapy study in high-risk PCa patients treated with contemporary radiation and androgen suppression. Our results indicate that moderate HFRT to high risk PCa patients is equally well tolerated as CFRT at 2 years. Clinical trial information: NCT01444820.