BACKGROUND: Due to the paucity of suitable donor organs for lung allotransplantation, a number of techniques have been developed to improve the lung preservation. Ultrastructural studies of the morphologic changes of the flushing, preservation and reperfusion injury in donor lungs have rarely been reported. METHODS: Adult dogs (n=46) were matched as donors and recipients for the single lung transplantation. The donor lungs were preserved after flushing with preservation solution and transplanted after 20-hours of preservation at 10degrees C. Ult rastructural features of the lung were examined after flushing, preservation and 2 hours after lung transplantation (reperfusion) respectively. RESULTS: Electron microscopy after flushing showed focal alveolar collapse and mild swelling of type I epithelial cells. After preservation both type I epithelial cells and endothelial cells were swollen and destroyed focally. The endothelial cells showed protrusion of tactile-like structure into the lumina, bleb or vacuole of the cytoplasm. After reperfusion the lung tissue showed fibrin material in the alveoli, prominent type I epithelial cell swelling with fragmented cytoplasmic debris and marked endothelial cell swelling with vacuoles or tactile-like projections. The alveolar macrophages showed active phagocytosis. Scanning electron microscopic examination of the pulmonary parenchyma showed focally alveolar collapse and focal consolidation after the preservation and more prominent changes after the reperfusion procedure. CONCLUSION: The ultrastructural changes associated with flushing were mild in severity, the donor lungs were injured during the preservation, and further damage was occurred during the reperfusion. The reperfusion injury resulted in prominent pulmonary parenchymal alterations with a pattern of acute lung injury.