BACKGROUND: Several studies have shown the efficacy of endocrine therapy in combination with radiotherapy in high-risk prostate cancer. To assess the effect of radiotherapy, we did an open phase III study comparing endocrine therapy with and without local radiotherapy, followed by castration on progression. METHODS: This randomised trial included men from 47 centres in Norway, Sweden, and Denmark. Between February, 1996, and December, 2002, 875 patients with locally advanced prostate cancer (T3; 78%; PSA<70; N0; M0) were centrally randomly assigned by computer to endocrine treatment alone (3 months of total androgen blockade followed by continuous endocrine treatment using flutamide; 439 patients), or to the same endocrine treatment combined with radiotherapy (436 patients). The primary endpoint was prostate-cancer-specific survival, and analysis was by intention to treat. This study is registered as an international standard randomised controlled trial, number ISRCTN01534787. FINDINGS: After a median follow-up of 7.6 years, 79 men in the endocrine alone group and 37 men in the endocrine plus radiotherapy group had died of prostate cancer. The cumulative incidence at 10 years for prostate-cancer-specific mortality was 23.9% in the endocrine alone group and 11.9% in the endocrine plus radiotherapy group (difference 12.0%, 95% CI 4.9-19.1%), for a relative risk of 0.44 (0.30-0.66). At 10 years, the cumulative incidence for overall mortality was 39.4% in the endocrine alone group and 29.6% in the endocrine plus radiotherapy group (difference 9.8%, 0.8-18.8%), for a relative risk of 0.68 (0.52-0.89). Cumulative incidence at 10 years for PSA recurrence was substantially higher in men in the endocrine-alone group (74.7%vs 25.9%, p<0.0001; HR 0.16; 0.12-0.20). After 5 years, urinary, rectal, and sexual problems were slightly more frequent in the endocrine plus radiotherapy group. INTERPRETATION: In patients with locally advanced or high-risk local prostate cancer, addition of local radiotherapy to endocrine treatment halved the 10-year prostate-cancer-specific mortality, and substantially decreased overall mortality with fully acceptable risk of side-effects compared with endocrine treatment alone. In the light of these data, endocrine treatment plus radiotherapy should be the new standard.