PURPOSE: To evaluate the incidence and prognostic factors of treatment-related pneumonitis in non-small-cell lung cancer (NSCLC) patients treated with intensity modulated radiation therapy (IMRT). MATERIALS AND METHODS: One-hundred-five patients with NSCLC treated with IMRT between 1 August 2004 and 30 November 2006 were analyzed retrospectively. The mean age of patients was 62.9 years, and squamous carcinomas were confirmed in 81 patients (77%). Sixty-six patients (62.9%) were classified as stage III, and 59 patients had lesions in the right lung. Twenty-seven patients were treated with a dose of 3,060 cGy preoperatively, and 10 patients were given a dose of 5,040 cGy postoperatively. Sixty-eight patients received a dose of 7,020 cGy for curative intent. Sixty-eight patients were treated with the use of the CORVUS planning system and 37 patients were treated with the use of the ECLIPSE planning system. RESULTS: Of 105 patients, 21 patients (20%) had abnormal radiological findings, but only seven patients (6.7%) required treatment for radiation pneumonitis. Six of the seven patients had other serious lesions, including a bronchioesophageal fistula (one patient), recurrence in the treatment field (two patients), brain metastasis (one patient) and lung-to-lung metastasis (two patients); all of these patients died within 19 months after radiation treatment. Sixteen patients (23.5%) that received planning with the CORVUS system had abnormal lung findings. Five patients (13.5%) had abnormal lung findings with the use of the ECLIPSE planning system. Other prognostic factors such as perioperative radiation therapy, a volume over 10% of the V20 volume in the right lung, were also statistically significant. CONCLUSION: This retrospective analysis suggests that IMRT could be a beneficial treatment modality for the reduction of radiation pneumonitis in NSCLC patients. However, the higher incidence of abnormal radiological findings in perioperative patients treated with relatively lower doses (3,060~5,040 cGy) suggest the need for judicious treatment planning in preoperative or postoperative treatment.