PURPOSE: To describe soft-tissue radiographic and high-resolution CT findings of radiation-induced lung injury of rabbit over time and to correlate them with pathologic findings. MATERIALS AND METHODS: 15 rabbits were irradiated in the right lung with one fraction of 2000 cGy. After 4, 6, 12, 20, 24 weeks, 3 rabbits in each group were sacrificed and soft-tissue radiographs and high-resolution CT of their lung tissue were obtained. Radiological findings were correlated with pathologic findings. RESULTS: On soft- tissue radiogram, radiation pneumonitis shown as consolidation with air-bronchogram occurred in 3 cases after 6 weeks, and in 1 case after 12 weeks of irradiation. In addition, pneumonic consolidation with adjacent pleural contraction was seen in 2 cases after 12 weeks of irradiation. Fibrotic changes indicated by decreased volume occurred after 20 weeks and combined bronchiectatic change and bronchial wall thickening appeared after 20 weeks(N=1), and 24 weeks(N=3). HRCT findings of radiation pneumonitis were homogeneous, increased attenuation after 4 weeks(N=3), 6 and 12 weeks (each N=I), patchy consolidation after 6 and 12 weeks(each N=2), discrete consolidation after 12, 20 and 24 weeks(each N=I) and solid consolidation after 20 and 24 weeks(each N=2). Pathologically radiation pneumonitis and pulmonary congestion were seen after 4 and 6 weeks. After 6 weeks, collagen and reticulin fibers were detected along alveolar wall. Mixed radiation pneumonitis and fibrosis were detected after 12 weeks. 20 weeks after irradiation, fibrosis was well defined in interstitium and in 24 weeks, decreased number of alveoli and thickening of bronchial wall were defined. CONCLUSION: Radiation pneumonitis was provoked 4 weeks after irradiation on rabbit lung and progressed into radiation fibrosis 20 weeks after irradiation on soft-tissue radiographs and high-resolution CT. High-resolution CT is more precise in detecting early radiation pneumonitis and detailed pathologic findings.