PURPOSE: To investigate ultrastructural changes of the mouse lung induced by whole lung gamma irradiation and to evaluate the effect of prophylactic administration of steroid against acute lung injury. MATERIALS AND METHODS: One hundred and twenty ICR mice were used and whole lung was irradiated with telecobalt machine. Whole lung doses were 8 and 12Gy, and 10mg of methyl prednisolone was administrated intraperitoneally for two and four weeks. At the end of the observation period, mice were sacrificed by cervical dislocation. The lungs were removed and fixed inflated. Histopathological examination of acute radiation injuries were performed by light microscopic and transmission electron microscopic examination. RESULTS: Control group with 8Gy is characterized by damage to the type I pneumocyte and the endothelial cell of the capillary, edema of alveolar wall and interstitium, and fibroblast proliferation. Control group with 12Gy is characterized by more severe degree of type I pneumocyte damage and more prominant inflammatory cell infiltration. Destructed cell debris within the alveolar space were also noted. After steroid administration, 8Gy experimental group showed decreased degree of inflammatory reactions but fibroblast proliferation and basal lamina damages were unchanged. Experimental group with 12Gy showed lesser degree of inflammatory reactions similar to changes of 8Gy experimental group. CONCLUSION: These studies suggest that the degree of interstitial edema and inflammatory changes were related to radiation dose but proliferation of the fibroblast and structural changes of basal lamina were not related to radiat- ion dose. Experimental administration of steroid for 2 to 4 weeks after whole lung irradiation suggest that steroid can suppress alveolar and endot- helial damages induced by whole lung irradiation but proliferation of the fibroblast and structural changes of basal lamina were not related to administration of steroid.