The crucial event in the pathogenesis of sinus empyema is the ostial obstruction. On the basis of recent reports, some consequences of the obstruction on the local bacterial-host interrelationship have been outlined. The anaerobic gas environment of the empyema is one quality important to the selection of bacteria but probably also to the efficiency of the antibacterial activity of the granulocytes. Release of proteolytic enzymes from neutrophilic granulocytes in the sealed sinus may jeopardize the bacterial engulfment due to proteolytic degradation of opsonins. A longstanding exposure of the sinus mucosa to uninhibited proteolytic enzymes may explain the irreversible lesion of the mucosa in chronic sinusitis.