Fluid signal in the mastoid can be an incidental finding on T2-weighted magnetic resonance imaging and often is interpreted as mastoiditis by radiologists. This study examines 28 consecutive cases of such erroneously diagnosed "mastoiditis" and documents the presence or absence of otologic symptoms and clinical signs. We found a very low prevalence of otologic symptoms or pathology and no cases of mastoiditis in these patients, and we determined that magnetic resonance imaging is not an effective screening modality for mastoiditis. We also reviewed the literature on current diagnostic criteria for mastoiditis and propose alternative terminology to replace the use of excessively alarming terms such as "mastoiditis" unless they apply to that specific clinical diagnosis.