The major clinical experiences using fast neutrons for the treatment of locally advanced prostatic carcinomas are reviewed. In all trials to date, there is evidence that treatment regimens employing a component of fast neutrons achieve results that equal or surpass those obtainable with conventional megavoltage external beam irradiation for comparable groups of patients. Late complications of neutron treatment have not exceeded the complication rate expected by photon irradiation. The structure of the current Radiation Therapy Oncology Group phase III randomized trial comparing neutron and photon treatment of patients with stages B2, C, and D1 disease is discussed.