PURPOSE: The determine the chest CT findings in breast cancer patients who have undergone postoperative irradiation. MATERIALS AND METHODS: The chest CT findings in 36 female patients who underwent breast surgery and radiotherapy between May 1996 and March 2000 were retrospectively analysed. Prior to radiotherapy, baseline chest CT depicted normal parenchyma in all cases. In 11 patients, the ipsilateral breast and chest wall were irradiated using opposed tangential fields, while 25 were treated by the four fields method (opposed tangential fields plus anterior and posterior supraclavicular/high axillary fields), with a total dose of 5040-5400 cGy for 5-9 weeks. RESULTS: CT after radiotherapy demonstrated reticular opacity (n=24), perpendicular linear opacity (n=15), traction bronchiectasis (n=7), consolidation (n=6), ground glass attenuation (n=3), pathologic rib fractures (n=3), pleural effusion (n=2), and pleural thickening (n=1), while in five patients no abnormality was observed. In addition, in the anterolateral lung area of 23 (64%) of 36 patients who underwent tangential beam irradiation, CT demonstrated peripheral opacities. When supraclavicular and axillary portals were used, radiation-induced lung changes mostly occurred at the apex of the lung (n=24). Chest radiographs were abnormal in 26 patients and normal in ten; in five of these ten, CT demonstrated reticular opacity. CONCLUSION: Depending on the irradiation CT findings of radiation-induced lung injury in breast cancer include areas of increased opacity with or without fibrosis, in apical and/or anterior subpleural regions. CT may help differentiate radiation-induced parenchymal change from superimposed or combined lung disease.