The intention of this research was to define the predictive factors for acute esophagitis (AE) in lung cancer patients treated with concurrent chemotherapy and three-dimensional conformal radiotherapy. The data for 72 lung cancer patients treated with concurrent chemoradiotherapy between 2008 and 2010 were prospectively evaluated. Mean lung dose, mean dose of esophagus, volume of esophagus irradiated and percentage of esophagus volume treated were analysed according to esophagitis grades. The mean esophageal dose was associated with an increased risk of esophageal toxicity (Kruskal-Wallis test, P < 0.001). However, the mean lung dose and the volume of esophagus irradiated were not associated with an increased risk of esophageal toxicity (Kruskal-Wallis test, P = 0.50 and P = 0.41, respectively). The mean radiation dose received by the esophagus was found to be highly correlated with the duration of Grade 2 esophagitis (Spearman test, r = 0.82, P < 0.001). The mean dose of esophagus >= 28 Gy showed statistical significance with respect to AE Grade 2 or worse (receiver operating characteristic curve analysis, 95% CI, 0.929-1.014). In conclusion, the mean esophageal dose was significantly associated with a risk of esophageal toxicity in patients with lung cancer treated with concurrent radiotherapy and chemotherapy.