Background We report the long-term results of a trial of immediate postoperative irradiation versus a wait-and-see policy in patients with prostate cancer extending beyond the prostate, to confi rm whether previously reported progression-free survival was sustained. Methods This randomised, phase 3, controlled trial recruited patients aged 75 years or younger with untreated cT0-3 prostate cancer (WHO performance status 0 or 1) from 37 institutions across Europe. Eligible patients were randomly assigned centrally (1:1) to postoperative irradiation (60 Gy of conventional irradiation to the surgical bed for 6 weeks) or to a wait-and-see policy until biochemical progression (increase in prostate-specifi c antigen >0.2 μg/L confi rmed twice at least 2 weeks apart). We analysed the primary endpoint, biochemical progression-free survival, by intention to treat (two-sided test for diff erence at α=0.05, adjusted for one interim analysis) and did exploratory analyses of heterogeneity of eff ect. This trial is registered with ClinicalTrials.gov, number NCT00002511. Findings 1005 patients were randomly assigned to a wait-and-see policy (n=503) or postoperative irradiation (n=502) and were followed up for a median of 10·6 years (range 2 months to 16·6 years). Postoperative irradiation signifi cantly improved biochemical progression-free survival compared with the wait-and-see policy (198 [39·4%] of 502 patients in postoperative irradiation group vs 311 [61·8%] of 503 patients in wait-and-see group had biochemical or clinical progression or died; HR 0·49 [95% CI 0·41-0·59]; p<0·0001). Late adverse eff ects (any type of any grade) were more frequent in the postoperative irradiation group than in the wait-and-see group (10 year cumulative incidence 70·8% [66·6-75·0] vs 59·7% [55·3-64·1]; p=0.001). Interpretation Results at median follow-up of 10·6 years show that conventional postoperative irradiation signifi cantly improves biochemical progression-free survival and local control compared with a wait-and-see policy, supporting results at 5 year follow-up; however, improvements in clinical progression-free survival were not maintained. Exploratory analyses suggest that postoperative irradiation might improve clinical progression-free survival in patients younger than 70 years and in those with positive surgical margins, but could have a detrimental eff ect in patients aged 70 years or older.