The authors present two clinical studies performed in the ENT departments of two Belgian Universities. A total of 248 patients with mucoviscidosis (cystic fibrosis, CF) were assessed by means of nasal endoscopy. One hundred eighteen underwent computed tomography of the paranasal sinuses (CT) and 55 were endoscopically operated. This allowed the observation of different clinical patterns of rhinosinusitis: mucopyosinusitis (pseudomucocele) of the maxillary antrum with bulging of the lateral nasal wall (LNW), nasal polyposis with erosion of the LNW, and chronic purulent rhinosinusitis with an isolated prominent uncinate process. The treatment of those patients could be tailored to the individual clinical pattern. Medical therapy consisted of systemic antibiotics and topical drugs delivered by sprays or by lavages with a nose can. Surgery was mainly aimed at removing the massive polyposis when it interfered with the daily life activities. The use of the endoscope enabled to perform safely more extensive procedures resulting in a lower recurrence rate. In patients with chronic rhinosinusitis without polyposis, yet presenting ostiomeatal obstruction, a limited and more functional endoscopic surgery was indicated in order to restore some drainage and to improve the penetration of topical drugs into the affected sinus. A short addendum presents two studies: one about genetics and the other about prevalence of middle ear disease in CF. The first concluded that no clear correlation was found between DF508 (the most common CF mutation) and nasal polyposis. The second revealed that in contrast with the extremely high prevalence of sinus problems, there was no clear evidence of an increased prevalence of middle ear disease in CF.