Serum levels of testosterone, dihydrotestosterone, and follicle-stimulating and luteinizing hormones were determined prospectively in 59 patients with carcinoma of the prostate treated curatively with external beam radiotherapy. Hormone levels were determined before the initiation of therapy and up to 2 years following completion of therapy. Testosterone levels remained unchanged but dihydrotestosterone levels decreased slightly. Follicle-stimulating and luteinizing hormone levels increased significantly during therapy and remained elevated for up to 2 years after therapy. These findings are consistent with low dose irradiation of the testis.