A prospective analysis of 666 patients was performed to examine the prevalence of radiologic abnormalities of the paranasal sinuses in asymptomatic adults. The initial sample group included 1000 patients who were referred for cranial computed tomographic scans for conditions such as head injuries and seizures. Patients in whom there was clinical suspicion of sinus disease were excluded from the study. A questionnaire was completed by each patient and cranial computed tomography, including magnetic resonance imaging of the paranasal sinuses, was performed. Abnormality of one or more of the paranasal sinuses was reported in 42.5% of scans. Mucosal thickening in the ethmoid sinus was the abnormality most often identified. The high frequency of reported radiologic abnormalities in asymptomatic patients highlights the importance of correlation with the clinical presentation when interpreting computed tomographic scans of the paranasal sinuses.