['The primary cause of COPD and lung cancer is smoking. Thus, patients with COPD frequently have lung cancer that often is inoperable. Stereotactic body radiation therapy (SBRT) is anticipated to be the standard of care for inoperable early stage non-small cell lung cancer. The most critical toxicity following SBRT is radiation pneumonitis (RP). We analyzed predictive factors for RP following SBRT and investigated the degree and occurrence of RP in patients with severe COPD.', 'We retrospectively evaluated 265 lung tumors treated with SBRT between 2005 and 2010 with a minimum follow-up of 6 months. Predictive factors for RP, including GOLD (Global Initiative for Chronic Obstructive Lung Disease) stage and pack-years smoked, were evaluated by univariate and multivariate analyses. RP was graded according to the Common Terminology Criteria for Adverse Events version 3.0 scale.', 'Median follow-up was 19.2 months (range, 6.0-72.0 months). RP grades of 0, 1, 2, 3, 4, and 5 occurred in 101, 102, 49, 12, 0, and one of these patients, respectively. Multivariate analysis revealed that high normal lung volume receiving ≥ 20 Gy, fewer pack-years smoked, and high total dose were significant predictive factors for RP ≥ grade 1, and high normal lung volume receiving 20 Gy, fewer pack-years smoked, and a history of lung resection were predictive for RP ≥ grade 2. RP in patients with more severe COPD was relatively milder than in patients with normal lung function and with mild COPD. Pack-year scales were significantly correlated with GOLD stage.'] Is severe COPD correlated with mild radiation pneumonitis following stereotactic body radiotherapy?