Orbital decompression and, in some cases, decompression of the optic nerve are the principal surgical procedures used for treatment of moderate or severe Graves' orbitopathy (GO). Histological examination of the surgical specimens of the ethmoid revealed a wide spectrum of inflammatory mucosal changes. The charts of 68 GO patients (55 female and 13 male; age range: 14 - 85 years) were retrospectively reviewed. Lund - Mackay scores were calculated for each patient based on findings of pre-operative computer tomography (CT) sinus scans, and the incidence of histological changes associated with polypoid and eosinophilic inflammation was assessed. Files did not reveal any evidence of chronic rhinosinusitis with or without nasal polyps based on endoscopic findings. Sinus opacification on CT (of any extent) was found in 20 out of the 68 patients (29.4%). On histological exam, histological changes of the sinus mucosa indicative for chronic rhinosinusitis were found in 31 out of the 68 GO patients (45.5%). A histological examination of the sinus mucosa indicative for chronic polypoid inflammation was present in 25 patients. Fourteen out of these 25 patients showed mucosal tissue eosinophilia on histology. Six patients had mucosal changes suggesting chronic non-polypoid inflammation with tissue eosinophilia on histological exam. The incidence of chronic rhinosinusitis in individuals without GO ranges between 10 and 15%. The incidence of histological changes of the sinus mucosa indicative for chronic rhinosinusitis described in this investigation suggests that chronic inflammatory disease is considerably more frequent in GO patients, when compared to the incidence of chronic rhinosinusitis in individuals without GO. Additionally, our data underline that CT imaging of the paranasal sinuses underestimates (29.4%) the incidence of inflammatory changes of the sinus mucosa (45.5%) of any extent in GO patients.