With the introduction of X-rays of higher energy that have higher penetrability, it has become possible to treat the deep-seated tumor with increased local control rate. But at the same time it has incrased the damage to the deep seated organs, especially to the lung which is known to be the less radiotolerable tissue in the body. This study analyses the 66 patients who were exposed to the irradiation of the lung, and examines the development of radiation pneumonitis and its related factors. The results of the study are summarized as folows : 1. The 66 patients were consisted of 40 cases of lung cancer, 15 cases of breast cancer and 11 cases of mediastinal tumors. There were 37 males and 29 females with the male to female ratio 1.3 : 1. A male to female ratio in the lung cancer was 3 : 1. 2. Among 66 patients, 26 patients (39%) developed the radiographical changes of acute radiation pneumonitis and 13 out of 26 patients (50%) showed the clinical features of acute radiation pneumonitis. 3. The onest of acute radiation pneumonitis ranged from 10 days to 6 months after the completion of radiotherapy. 4. There was a statistically significant close relationship between the development of radiation pneumonitis and the radiation dose. 5. As the irradiated lung volume increased, the development of radiation pneumonitis increased. But the statistical significance was not strong. 6. The increased incidence of radiation pneumonitis was observed when the chemotherapy was given before or concomittantly with radiotherapy. 7. There was no significant correlation between the development of radiation pneumonitis and the age, smoking and the presence of underlying lung disease.