Background: Causes of osteitis in chronic rhinosinusitis (CRS) other than previous surgery are poorly defined. Patients with eosi- nophilic CRS (ECRS) have more severe disease and poorer outcomes despite repeated surgery. Associations between osteitis and markers of ECRS are not well described. Methods: A cross-sectional study of CRS patients undergoing sinus surgery was conducted. Osteitis was scored radiologically using previously published measures. Associations between osteitis and histopathology, symptoms, endoscopy, CT mucosal score and seromarkers were analyzed. Results: Eighty-eight patients were assessed of whom forty-five had osteitis. Patients undergoing revision surgery recorded higher osteitis scores. Patients with mucosal eosinophilia had higher osteitis score than those without. Patients with osteitis had higher serum eosinophil. Similar relationships were also found in primary surgery. Osteitis was associated with endoscopic and radiologic, but not symptomatic disease severity. Conclusions: Osteitis is associated with tissue and serum eosinophilia in both patients with and without prior surgery. Patients with these features may benefit from post-operative corticosteroid therapy to prevent osteitis.