Objective To analyze the incidence and related factors of acute radiation pneumonitis (RP) in patients with non-small cell lung cancer (NSCLC) who underwent three-dimensional conformal ra-diotherapy with concurrent chemotherapy. Methods From January 2006 to October 2008, 90 NSCLC pa-tients treated with concurrent chemoradiotherapy were retrospectively reviewed. The clinical characteristics and treatment plan parameters from dose-volume histogram (DVH) were analyzed. Results Grade 1, 2,3, 4 and 5 RP was found in 32.2%, 30.0%, 5.6%, 0% and 1.1% of the patients. Primary tumor loca-tion was the only significant clinical factor in univariate analysis. And significant differences were found when the groups was divided by the mean lung dose (MLD) of 17 Gy, V_(5) of 47% ,V_(10) of 36% ,V_(20) of 24%,V_(30) of 23% and V_(40) of 22%, respectively. Multivariate analysis showed the MLD was the only independent predictive factor of RP. Conclusions Some factors may contribute to the RP. MLD should be set below 17 Gy in three-dimensional conformal radiation, especially when the tumor locates in the lower lobe.