Over 200 patients have been entered in five studies investigating the use of fast neutron radiation therapy in the treatment of non-small-cell carcinomas of the lung since 1983. The results of these studies have been inconsistent. Most studies did not show survival rates or local control advantages over standard photon radiation therapy. Side effects from studies employing mixed photon-neutron treatment plans or clinically oriented, high-energy cyclotrons were seen to be comparable to those of standard courses of radiation therapy, representing a considerable improvement over those studies utilizing low-energy cyclotrons for a full course of radiation therapy, which resulted in unacceptably high complication rates. A new phase III study utilizing high-energy isocentric neutron beams has been designed and implemented, and over 100 patients have been entered to date. The current status of fast neutron radiation therapy in the treatment of non-small-cell lung cancer is reviewed.