Purpose : Radiation pneumonitis and fibrosis are common complications of radiation therapy of the chest. Sixty-six patients with lung cancer were studied to evaluate the radiation injury according to radiation dose(total and estimated single dose), time after completion of radiation, overall treatment time, and combined chemotheraphy and chest surgery. Materials and Methods : All 66 patients received fractionated radiotherapy (180 to 300cGy/day) to the thorax with total 1000 to 6660cGy encompassing primary site and regional lymphatics. Results : Radiation pneumonitis was developed in 40(60%) of 66 patients. Radiation pneumonitis occurred average 11 weeks after completion of therapy in 14(44%) of 32 patients below 4000cGy, and 6 weeks in 26(76%) of 34 patients above 4000cGy. Radiation pneumonitis occurred more frequently when the estimated single dose was over 1100 units than below 1100 units. Radiation pneumonitis was more frequent when the overall treatment time was over 30 days than within 30 days. All 10 patients undergone lobectomy or pneumonectomy developed radiation pneumonitis. Conclusion : The incidence of radiation induced lung injury is variably dependant on radiation dose, time from completion of radiation therapy, overall treatment time, and history of surgery such as lobectomy or pneumonectomy before radiation therapy.