To determine how soon radiation-induced lung injury is detectable, to compare the CT findings with those on chest radiographs, and to observe the appearance of the abnormality during the acute phase, we performed 83 CT studies in 17 radiotherapy patients at relatively short intervals. All 17 patients received fractionated radiotherapy to the thorax with a large irradiated lung volume. The CT findings were variable; pulmonary infiltrates were homogeneous, patchy, or discrete. CT abnormalities were evident in 15 of 17 cases within 16 weeks after radiotherapy; in 13 of these it was detected within 4 weeks. In three of these 15 cases, no abnormality was detected on chest radiographs, and in three other cases, the change was observed much later on chest radiographs than on CT scans. In the other nine cases, abnormalities were detected simultaneously on CT scans and chest radiographs. In four cases, extensive radiation pneumonitis was observed on CT, but in two of these, the change was misdiagnosed on the chest radiograph. We conclude that CT is useful in the detection of acute radiation-induced pulmonary disease.