In the vast majority of cases infections of the paranasal sinus system are rhinogenic. Usually these spread via the middle nasal meatus and the anterior ethmoid to the dependent larger sinuses, especially to the frontal and/or maxillary sinus. If a sinusitis does not heal or is constantly recurring, a focus of infection has remained in a stenotic cleft of the lateral nasal wall, irritating nasal function and where from infection time and again may spread to the dependent sinuses. These Infection foci may be very circumscribed and limited, and not always must present with the typical triad of sinusitis symptoms: pathological secretion, nasal obstruction and cephalgia. Frequently only one of these symptoms prevails. By the means of nasal endoscopy and polytomography these foci can exactly be localized. After clearing the infection foci, which easily can be achieved under endoscopic guidance, mucosal function usually is restored and the dependent larger sinuses heal without having been touched.