Objective: To investigate the question of whether there is a difference in retronasal olfactory function between patients suffering from chronic rhinosinusitis with nasal polyposis (NP) and healthy controls. This question was based on the clinical observation that many of these patients present with smell loss without complaining about loss of the appreciation of foods. Study Design: Open prospective study comparing symptomatic patients with healthy controls. Methods: A total of 56 healthy volunteers and 42 NP patients were tested for orthonasal and retronasal odor identification. All subjects received detailed nasal endoscopy; NP was staged according to the Malm classification. Patients rated their olfactory function on visual analogue scales. Orthonasal testing was performed using the "Sniffin' Sticks" test kit. Retronasal testing was evaluated with odorized powders applied to the oral cavity. In both tests, subjects were asked to identify 10 items using a forced choice paradigm. Results: Overall, odor identification was better in controls compared with NP patients (P < .001). Although controls exhibited no difference between orthonasal and retronasal smelling (P = .26), in NP patients, olfactory function was significantly better when odors were applied through the retronasal route (P < .001). Ratings of general olfactory abilities correlated with retronasal and orthonasal olfactory function in NP patients (P < .001) but not in healthy controls (P = .34) Conclusion: Better retronasal than orthonasal olfactory function seems to be associated with the presence of mechanical obstruction in the anterior portion of the olfactory cleft. In turn, these data indicate that olfactory loss in NP seems to be caused by regional mechanical or inflammatory factors.