An 80-year-old man was admitted to our division because of hemosputum, cough, and chest pain for three months. A chest roentgenogram, chest CT scanning, and bronchoscopic examinations revealed adenocarcinoma of the lung with atelectasis of the right upper lobe. The patient developed radiation pneumonitis after receiving radiation therapy (5,100 cGy) for lung cancer. At the same time, the right upper lobe atelectasis improved and movement of infiltrates consistent with radiation pneumonitis to the middle lung fields occurred. A chest roentgenogram taken when the atelectasis had improved revealed the absence of pneumonitis shadows in the right upper lobe, suggesting that the atelectatic lung escaped radiation pneumonitis.