We conducted a prospective exploratory study to investigate the relationship between radiation pneumonitis (RP) and transforming growth factor-β1 (TGF-β1) in exhaled breath condensate (EBC).The inclusion criteria were: patients who 1) received thoracic radiotherapy (RT) for lung cancer, 2) were aged ≥20 years, and 3) provided written informed consent. EBC was collected before and 1 month after RT. TGF-β1 levels in EBC were measured using an enzyme-linked immunosorbent assay. We evaluated RP using the Common Terminology Criteria for Adverse Events v4 and analyzed the relationship between grade (G) 2 RP and TGF-β1 levels in EBC.Ten patients were enrolled [median age, 75 years (range=60-81 years)], and none of them had interstitial lung disease. Conventional fractionation, accelerated hyperfractionation, hypofractionation, and stereotactic ablative fractionation were used in four, one, two, and three patients, respectively. G1 and G2 RP were observed in five patients each; no G3-G5 RP occurred. The median TGF-β1 levels in EBC before and 1 month after RT were 79.1 pg/ml (0.1-563.7 pg/ml) and 286.9 pg/ml (33.7-661.3 pg/ml), respectively. Of the seven patients with increased TGF-β1 levels in EBC 1 month after RT than before RT, five (71%) experienced G2 RP, whereas the remaining three patients with decreased TGF-β1 levels had G1 RP (p=0.083, one-sided Fisher's exact test).Increased TGF-β1 levels in EBC 1 month after RT might be promising for the detection of G2 RP.