The maximum dose of radiation which can be delivered to a tumor is limited by the tolerance of the surrounding normal tissues. The ability to identify patients at high or low risk of injury from radiation therapy might enable the clinician to tailor radiation doses in order to maximize efficacy and minimize risk. The cytokine transforming growth factor beta 1 (TGF beta 1) has been implicated in the development of normal tissue injury after irradiation in several organs, including the lung. Herein, the evidence supporting the role of TGF beta 1 in radiation-induced lung injury is reviewed. Using the treatment of non-small cell lung cancer as a model, we also discuss how it may be possible to identify patients at risk for this complication using measurements of plasma TGF beta 1, and how this information may be used in the future to adjust doses of radiation in the treatment of lung cancer.