Abstract Purpose : To determine, with a prospective, multicenter randomized study, whether fast neutron radiation therapy improves the outcome for patients with non-small cell lung cancer, as compared to conventional photon radiotherapy. Methods and Materials : From September 1986 to March 1991, a total of 200 patients with inoperable regional non-small cell lung cancer were randomized to 20.4 Gy in 12 fractions with neutrons versus 66 Gy in 33 fractions with photons. Inoperable patients with Radiation Therapy Oncology Group Stages I, II, III, or IV(MO) disease, Karnofsky Performance Score ≥ 70, and who had received no previous therapy for their non-small cell lung cancer were eligible for the study. Of the 200 patients randomized, a total of 193 patients, 99 on the neutron arm and 94 on the photon arm, were eligible for analysis. The two treatment groups were balanced with regards to prognostic factors. At the time of this analysis, the median at-risk follow-up was 33 months, with a minimum follow-up of 16 months. Results : No difference in overall survival was observed; however, there was a statistically significant improvement in survival for patients with squamous cell histology ( p = 0.02), and a trend toward improved survival for those with favorable prognostic factors (i.e., patients who were not T4, N3, and had no pleural effusion or weight loss > 5% from baseline) ( p = 0.15), favoring the neutron-treated group. With the exception of skin and subcutaneous changes, acute and late toxicity was similar in both arms. Conclusion : In selected patients with inoperable regional non-small cell lung cancer (e.g., squamous cell histology, favorable prognostic favtors), fast neutron irradiation provides a therapeutic benefit over conventional photon radiotherapy.