A 160-MeV proton beam has been modified to irradiate patients with localized tumors by using convention treatment schedules. This proton beam has the physical advantage of megavoltage x-rays of reducing the radiation dose to normal tissues adjacent to the tumor volume. A perineal proton technique used as boost therapy (2,000 to 2,500 rads) was evaluated in the definitive irradiation of 17 patients with localized prostatic carcinoma. This technique allows repeated daily treatment of the carefully defined target volume with a precision of +/- 2 mm. Total dose to the prostatic tumor, but not to the posterior rectum, has been increased by 500 to 700 rads. After 12 to 27 months of observation, no noteworthy rectal reaction has developed in a patient, easily managed urethral strictures have developed in two patients, and all but one are locally controlled.