Between March 1973 and May 1975, 82 patients with locally advanced pelvic tumors were treated with 50 MeVdleads to Be neutrons. Seventy-nine have been included for analysis. The clinical material included 51 patients with gynecologic cancers, 15 with colorectal adenocarcinomas, and 13 with prostatic adenocarcinomas. Three treatment approaches have been used: 1) neutrons only--160 rad mu gamma twice weekly, 2) neutron boost following 25 MV photon irradiation--160 rad mu gamma twice weekly, and 3) mixed beam--photons (200 rads) three times weekly and neutrons (65 rad mu gamma) twice weekly. The preliminary results with the mixed beam are superior to those obtained with neutrons only or a neutron boost. However, the cancer and have been treated to slightly higher equivalent doses than those treated with neutrons only. The superior results with the mixed beam may be related to a better volume distribution and/or five-times weekly fractionation.