The purpose of this study was to summarize the progress made using fast neutron irradiation in the treatment of prostate cancer at Wayne State University between 1991 and the year 2001. The results of three Phase II studies and one Phase III st udy involving nearly 700 patients is summarized in this paper. The Phase II studies weredose finding studies looking at doses of 15, 9, 10, and 11 nGy, respectively. The randomized protocol was a study of sequence looking at the results of treating patients with neutron first versus neutron radiation last. The results demonstrated that the best combination of tumor control probabilities and normal tissue complications was found in a mix of approximately 50% neutrons and 50% photons. Thus, the standard doses become 10 nGy and 40 Gy of photons. The randomized trial demonstrated that the sequence has significant importance and the disease-free survival was 93% for patients treated with neutrons first versus 73% for patients treated with neutrons last. There was no difference in the rate of acute or chronic complications. Finally, an analysis was performed demonstrating which patients may best benefit from the use of neutron irradiation. It was shown that patients with one, two, or three adverse risk factors had a significant improvement in disease-free survival when part of the treatment included neutron radiation versus standard photon radiation alone. Neutron radiation can be delivered safely with effort to see that it is superior to that which can be achieved by conformal photon irradiation by itself. Future work will be done to expand the role of neutron radiation in other clinical disease sites.