Objective: In this study, we investigated the incidence of radiation pneumonitis (RP) in non-small-cell lung cancer (NSCLC) patients undergoing helical tomotherapy (HT) and the clinical and dosimetric factors associated with it.Materials and Methods: We analyzed data from the treatment protocols of 62 NSCLC patients.The median total radiation dose was 64 Gy (range 57.6-66 Gy) at 1.8-2.2 Gy/fraction.Thirty-four of these patients underwent HT alone and 28 underwent HT in combination with chemotherapy.Treatment-related pneumonitis was graded according to the Common Terminology Criteria for Adverse Events, version 3.0.Results: We found that RP grades 1,2, 3 and 5 occurred in 29 (46.8%), 23 (37.1%), 8 (12.9%), and 2 (3.2%) patients,respectively.Using univariate analyses, we found that a grade ≥3 RP was associated with poor performance status (PS),age, planning target volume, mean lung dose, and relative V5 through V25, in increments of 5 Gy (P < 0.005).We determined that PS and V5V15 were the most significant factors associated with grade ≥3 RP using multivariate analysis.Conclusions: We found that poor PS and V5-V15 were the risk factors associated with grade ≥3 RP in NSCLC patients treated with HT.Thus, for NSCLC patients treated with HT, the volume of total lung with low-dose region (V5-V15) should be carefully regulated and the use of HT should be restricted in patients with Eastern Cooperative Oncology Group ≥2.