Between June, 1977, and April, 1983, the Radiation Therapy Oncology Group (RTOG) sponsored a phase III study comparing fast neutron radiotherapy as part of a mixed beam (neutron/photon) regimen with conventional photon (x-ray) radiotherapy for patients with locally advanced (Stages C and D1) adenocarcinoma of the prostate. A total of 91 analyzable patients were entered into the study with the two treatment groups balanced in regard to all major prognostic variables. The current analysis is for a median follow-up of 6.7 years (range 3.4-9.0 yrs.). The results are statistically significant in favor of the mixed-beam group of all parameters mentioned. At five years the freedom from local/regional relapse rate is 81 percent on the mixed-beam arm compared with 60 percent on the photon arm. The actuarial overall survival rate at five years is 70 percent on the mixed beam compared with 56 percent on the conventional photon arm. The determinantal survival at five years (which excluded death due to intercurrent disease in patients clinically free of cancer) was 82 percent on the mixed-beam arm compared with 61 percent on the photon arm. The type of therapy appeared to be the most important predictor of both local tumor control and patient survival in a step-wise Cox analysis. There was no difference in the treatment-related morbidity for the two patient groups. Mixed-beam therapy may be superior to standard photon radiotherapy for treatment of locally advanced prostate cancer.