BACKGROUND: We conducted a randomized, prospective trial comparing external irradiation with external irradiation plus goserelin (an agonist analogue of gonadotropin-releasing hormone that reduces testosterone secretion) in patients with locally advanced prostate cancer. METHODS: From 1987 to 1995, 415 patients with locally advanced prostate cancer were randomly assigned to receive radiotherapy alone or radiotherapy plus immediate treatment with goserelin. The patients had a median age of 71 years (range, 51 to 80). Patients in both groups received 50 Gy of radiation to the pelvis over a period of five weeks and an additional 20 Gy over an additional two weeks as a prostatic boost. Patients in the combined-treatment group received 3.6 mg of goserelin (Zoladex) subcutaneously every four weeks starting on the first day of irradiation and continuing for three years; those patients also received cyproterone acetate (150 mg orally per day) during the first month of treatment to inhibit the transient rise in testosterone associated with the administration of goserelin. RESULTS: Data were available for analysis on 401 patients. The median follow-up was 45 months. Kaplan-Meier estimates of overall survival at five years were 79 percent (95 percent confidence interval, 72 to 86 percent) in the combined-treatment group and 62 percent (95 percent confidence interval, 52 to 72 percent) in the radiotherapy group (P=0.001). The proportion of surviving patients who were free of disease at five years was 85 percent (95 percent confidence interval, 78 to 92 percent) in the combined-treatment group and 48 percent (95 percent confidence interval, 38 to 58 percent) in the radiotherapy group (P<0.001). CONCLUSIONS: Adjuvant treatment with goserelin, when started simultaneously with external irradiation, improves local control and survival in patients with locally advanced prostate cancer.