The aim of this study was to document patients with clinical mastoiditis who were reported to have mastoiditis by radiologists due to increased fluid signal intensity in the mastoid air cells on magnetic resonance imaging (MRI). Brain and temporal bone MRI reports between January 2004 and November 2009 were obtained from the radiology units of four different hospitals of the same Health Care Group. MRI reports for keyword mastoiditis and 406 patients were reported to have radiological mastoiditis on MRI due to increased fluid signal intensity. Otoscopic examination findings of 275 of these 406 radiological mastoiditis patients were documented and compared with MRI reports for clinical infectious otological disease. Forty-eight (17, 45%) (48/275) patients were found to have clinical otological disease on examination. The remainder of the patients (227/275, 82%) did not show any evidence of clinical otologic infectious disease. Of these 48 patients, 18 patients (37, 5%) had eustachian tube dysfunction, 13 patients (27%) had serous otitis media, nine patients (9%) had chronic otitis media, five patients (10%) had tympanosclerosis, and three patients (6%) had acute otitis media. The results of this study showed that MRI is not an effective diagnostic tool for mastoiditis. 82% of the MRI mastoiditis did not show clinical mastoiditis contradicting MRI reports. Fluid signal in the mastoid on MRI should not always be interpreted as mastoiditis by radiologists. Radiological mastoiditis does not necessarily point out to clinical mastoiditis.