Following a 30-year hiatus after Dr. Stone's work in the 1930's and 1940's, clinical trials with fast neutrons were restarted in the United States in 1972. Approximately 2500 patients have been treated with neutrons since that time. Three hundred and seven patients with squamous cell carcinomas of the head and neck were entered on an RTOG-coordinated randomized study comparing standard photon irradiation with mixed beam radiation therapy. No significant differences were noted in the local control, survival or complication rates. One hundred and sixty patients were entered on a randomized glioblastoma study. Although there were no significant differences in median survival, autopsy results showed greater tumor effect on the neutron-treated tumors. Twenty-six patients were treated for transitional cell carcinomas of the bladder with either preoperative mixed beam irradiation or mixed beam irradiation alone. Both the local control rates and survival rates compare favorably with photon radiation therapy. The future of fast neutron beam radiation therapy in the United States is discussed.