To test whether radiation-induced elevations of transforming growth factor-β1 (TGF-β1) during radiation therapy (RT) correlate with radiation-induced lung toxicity (RILT) in patients with non-small-cell lung cancer (NSCLC) and to evaluate the ability of mean lung dose (MLD) to improve the predictive power.Eligible patients included those with Stage I–III NSCLC treated with RT with or without chemotherapy. Platelet-poor plasma was obtained pre-RT and at 4–5 weeks (40–50 Gy) during RT. TGF-β1 was measured using an enzyme-linked immunosorbent assay. The primary endpoint was ≥ Grade 2 RILT. Mann-Whitney U test, logistic regression, and chi-square were used for statistical analysis.A total of 165 patients were enrolled in this study. The median radiation dose was 60 Gy, and the median MLD was 15.3 Gy. Twenty-nine patients (17.6%) experienced RILT. The incidence of RILT was 46.2% in patients with a TGF-β1 ratio > 1 vs. 7.9% in patients with a TGF-β1 ratio ≤ 1 (p < 0.001), and it was 42.9% if MLD > 20 Gy vs. 17.4% if MLD ≤ 20 Gy (p = 0.024). The incidence was 4.3% in patients with a TGF-β1 ratio ≤ 1 and MLD ≤ 20 Gy, 47.4% in those with a TGF-β1 ratio >1 or MLD > 20 Gy, and 66.7% in those with a TGF-β1 ratio >1 and MLD > 20 Gy (p < 0.001).Radiation-induced elevation of plasma TGF-β1 level during RT is predictive of RILT. The combination of TGF- β1 and MLD may help stratify the patients for their risk of RILT.