Long-term androgen deprivation therapy (ADT) combined with radiotherapy (RT) is a standard treatment for patients with localized high-risk prostate cancer (HRPC). However, the optimal duration of ADT is not yet defined. The aim of this superiority randomized trial was to compare outcomes of RT combined with either 36 or 18 mo of ADT. From October 2000 to January 2008, 630 patients with HRPC were randomized, 310 to pelvic and prostate RT combined with 36 mo (long arm) and 320 to the same RT with 18 mo (short arm) of ADT. Overall survival (OS) and quality of life (QoL) were primary end points. OS rates were compared with Cox Regression model and QoL data were analyzed through mixed linear model. With a median follow-up of 9.4 yr, 290 patients had died (147 long arm vs 143 short arm). The 5-yr OS rates (95% confidence interval) were 91% for long arm (88-95%) and 86% for short arm (83-90%), p=0.07. QoL analysis showed a significant difference (p<0.001) in six scales and 13 items favoring 18 mo ADT with two of them presenting a clinically relevant difference in mean scores of ≥10 points. In localized HRPC, our results support that 36 mo is not superior to 18 mo of ADT. ADT combined with RT can potentially be reduced to 18 mo in selected men without compromising survival or QoL. Thus, 18 mo of ADT appears to represent a valid option in HRPC. In this study, we report outcomes from high-risk prostate cancer patients treated with radiotherapy and either 36 or 18 mo of androgen deprivation therapy. There was no difference in survival between the two groups, with the 18-mo group experiencing a better quality of life.