Purpose-To assess the impact of induction chemotherapy, and associated tumor shrinkage, on the subsequent radiation-related changes in pulmonary function and tumor response.Methods and Materials-As part of a prospective IRB-approved study, 91 evaluable patients treated definitively with thoracic RT for unresectable lung cancer were analyzed. The rates of RTassociated pulmonary toxicity and tumor response were compared in the patients with and without pre-RT chemotherapy. In the patients receiving induction chemotherapy, the rates of RT-associated pulmonary toxicity and tumor response were compared in the patients with and without a response (modified RECIST criteria) to the pre-RT chemotherapy. Comparisons of the rates of improvements in PFTs post-RT, dyspnea requiring steroids, and percent declines in PFTs post-RT were compared in patient subgroups using Fisher's exact test, analysis of variance, and linear or logistic regression.Results and conclusion-The use of pre-RT chemotherapy appears to increase the rate of radiation-induced pneumonitis (p=0.07-0.01), but has no consistent impact on changes in PFTs. The degree of induction-chemotherapy-associated tumor shrinkage is not associated with the rate of subsequent-RT-associated pulmonary toxicity. The degree of tumor response to chemotherapy is not related to the degree of tumor response to RT. Additional study is needed to better clarify this issue.