Despite extensive research in the treatment of maxillary sinusitis and its complications, the cause of mucosal damage is not clear. A series of maxillary sinus mucosal specimens were studied with regard to the relationship between inflammatory cells and mucosal damage. The results show that a large number of inflammatory cells are seen in the subepithelial layer and also between the cells of the epithelium. Clefts were seen on the level of the basal membrane and between the cells of the epithelium. Fibronectin was also demonstrated in maxillary sinus mucosa by use of direct enzyme labeled antibody technique. Noticeable changes of quantities of fibronectin occurred beneath the basal membrane of the epithelium in chronic maxillary sinusitis in which it has accompanied with infiltration of inflammatory cells. The present study suggest that the mucosal damage in chronic maxillary sinusitis is caused by inflammatory cells and fibronectin is a sensitive target of products released from inflammatory cells.