Background: It remains unclear whether patients with positive surgical margins or extracapsular extension benefit from adjuvant radiotherapy following radical prostatectomy. Objective: To compare the effectiveness and tolerability of adjuvant radiotherapy following radical prostatectomy. Design, setting, and participants: This was a randomised, open-label, parallel-group trial. A total of 250 patients were enrolled between April 2004 and October 2012 in eight Finnish hospitals, with pT2 with positive margins or pT3a, pN0, M0 cancer without seminal vesicle invasion. Intervention: A total of 126 patients received adjuvant radiotherapy at 66.6 Gy. Outcome measurements and statistical analysis: The primary endpoint was biochemical recurrence-free survival, which we analysed using the Kaplan-Meier method and Cox proportional hazard regression. Overall survival, cancer-specific survival, local recurrence, and adverse events were secondary endpoints. Results and limitations: The median follow-up time for patients who were alive when the follow-up ended was 9.3 yr in the adjuvant group and 8.6 yr in the observation group. The 10-yr survival for biochemical recurrence was 82% in the adjuvant group and