Radiologic findings of the 29 cases of radiation pneumonitis and fibrosis diagnosed by chest radiography at Seoul National University Hospital were evaluated and compared with clinical symptoms according to the passage of time after radiation therapy. The results were as follows : 1. The first radiographic signs of radiation pneumonitis and fibrosis were observed 7.6 weeks and 19.3 weeks after radiation therapy respectively. Especially in 8 cases of small cell ca., they were found 5.6 weeks and 10.4 weeks, appearing slightly earlier than those of 12 cases of squamous cell ca., of lung. 2. The appearing time and severity of the radiographic changes of radiation pneumonitis and fibrosis had no specific relationship with field size, tumor dose or time-does-fractionation factors of the treatment. 3. The most constant and characteristic radiological finding of radiation pneumonitis was the sharp and straight margin of the lesion, which was not that of normal anatomical structures of the lung. Other findings were diffuse haziness, indistinct normal pulmonary markings, alveolar and nodular densities, air-bronchograms and indistinctness of heart border. In radiation fibrosis stage, the lesion also had sharp and straight border in all cases and linear streaks radiating from the lesion was also characteristic and constant finding. Other findings were indistinctness of heart border, diaphargatic tenting and compensatory emphysema.