BACKGROUND: We did a randomised phase III trial comparing external irradiation alone and external irradiation combined with an analogue of luteinising-hormone releasing hormone (LHRH) to investigate the added value of long-term androgen suppression in locally advanced prostate cancer. METHODS: Between 1987 and 1995, 415 patients were randomly assigned radiotherapy alone or radiotherapy plus immediate androgen suppression. Eligible patients had T1-2 tumours of WHO grade 3 or T3-4 N0-1 M0 tumours; the median age of participants was 71 years (range 51-80). In both treatment groups, 50 Gy radiation was delivered to the pelvis over 5 weeks, and 20 Gy over 2 weeks as a prostatic boost. Goserelin (3.6 mg subcutaneously every 4 weeks) was started on the first day of irradiation and continued for 3 years; cyproterone acetate (150 mg orally) was given for 1 month starting 1 week before the first goserelin injection. The primary endpoint was clinical disease-free survival. Analyses were by intention to treat. FINDINGS: 412 patients had evaluable data, with median follow-up of 66 months (range 1-126). 5-year clinical disease-free survival was 40% (95% CI 32-48) in the radiotherapy-alone group and 74% (67-81) in the combined-treatment group (p=0.0001). 5-year overall survival was 62% (52-72) and 78% (72-84), respectively (p=0.0002) and 5-year specific survival 79% (72-86) and 94% (90-98). INTERPRETATION: Immediate androgen suppression with an LHRH analogue given during and for 3 years after external irradiation improves disease-free and overall survival of patients with locally advanced prostate cancer.