Platelet activating factor (PAP) has been known as implicating as one of potent inflammatory mediators and reported 0 be involved in inflammation and allergy. PAF induces ciliary dysfunction and epithelial damage of human paranasal sinus mucosa in vitro. However, several recent papers have reported that PAF may not readily damage the airway epithelium. The aim of this study was to investigate the ultrastructural evidence to elucidate the pathogenesis of epithelial damage induced by PAF. Sixteen $mu extrm{g}$ g of PAF was applied into the maxillary sinuses of 6 rabbits. Rabbits were divided into 2 subgroups along with time interval at 1st and 3rd experimental day, and sinus mucosae were taken for the histopathologic study using electron microscopy. At 1st day, epithelial cells showed no ultrastructural change. Ultrastructures of the cilia were well preserved. Subepithelial space showed no evidence of the infiltration of inflammatory cells. Intravascular platelet aggregation and swelling of endothelial cells were evident. At 3rd day, epithelial cells showed vacuolar degeneration. Fusion of cilia forming giant cilia and focal loss of cilia were evident. Eosinophils were infiltrated in subepithelial and intraepithelial space. Swelling of endothelial cells, and migration of inflammatory cells into the connective tissue were evident. This study implies that epithelial damage induced by PAF may be secondary to the cytotoxicity of mobilized eosinophils rather than direct cytotoxicity of PAF.