To evaluate the prevalence of ethmoid sinus abnormalities in adults without clinical history of sinusitis or allergic rhinitis, the brain CT scans of 156 patients were analyzed prospectively. In 17 cases (10.9%) the ethmoid labyrinth showed abnormalities. In most of these cases (88%) the ethmoid disease was localized to four or fewer cells. Men were more often affected than women (ratio, 2.5:1), and the prevalence of abnormalities was fairly similar across age groups.