PURPOSE: Extraprostatic disease will be manifest in a third of men after radical prostatectomy. We present the long-term followup of a randomized clinical trial of radiotherapy to reduce the risk of subsequent metastatic disease and death. MATERIALS AND METHODS: A total of 431 men with pT3N0M0 prostate cancer were randomized to 60 to 64 Gy adjuvant radiotherapy or observation. The primary study end point was metastasis-free survival. RESULTS: Of 425 eligible men 211 were randomized to observation and 214 to adjuvant radiation. Of those men under observation 70 ultimately received radiotherapy. Metastasis-free survival was significantly greater with radiotherapy (93 of 214 events on the radiotherapy arm vs 114 of 211 events on observation; HR 0.71; 95% CI 0.54, 0.94; p = 0.016). Survival improved significantly with adjuvant radiation (88 deaths of 214 on the radiotherapy arm vs 110 deaths of 211 on observation; HR 0.72; 95% CI 0.55, 0.96; p = 0.023). CONCLUSIONS: Adjuvant radiotherapy after radical prostatectomy for a man with pT3N0M0 prostate cancer significantly reduces the risk of metastasis and increases survival.