PURPOSE: Radiation-induced fibrotic mass might masquerade as a true hilar tumor mass on a plain chest radiograph. We attempted to differentiate radiation fibrosis from a true hilar tumor using only a plain radiograph. MATERIALS AND METHODS: Plain chest radiographs were obtained from seven patients who had developed radiation fibrosis simulating hilar mass after radiation therapy for lung cancer, and from 19 patients with lung cancer, a comparison group, who had not received radiation therapy. They were reviewed for the obliteration of the overlapped mediastinal and hilar anatomical silhouettes by the mass : pulmonary artery, heart or aorta border, and paraspinal line. RESULTS: All seven patients with radiation-induced fibrotic mass(bilateral lesion in twopatients) showed obliteration of all three overlapped anatomical silhouettes of the mediastinum and hilum on chestradiographs. in the comparison group of 19 patients with a true hilar mass, there was, however, no case which demonstrated obliteration of all landmarks. CONCLUSION: Obliteration of all anatomical landmarks at the hilum and mediastinum may be a helpful clue in the differential diagnosis of radiation-induced fibrotic mass from true hilarmass on a plain radiograph, particularly when clinical information on previous radiation therapy is unavailable.