PURPOSE: To evaluate incidentally observed thickened mucosa of paranasal sinuses on brain MRI of patients without evidence of sinusitis. MATERIALS AND METHODS: We reviewed brain MRI of 82 adults aged over 20 ; 45 were males and 37 were females. Brain axial MRI was obtained from the hard palate with 8mm thickness and 2mm gap. The mucosal thickness of incidentally observed paranasal sinuses seen on brain MRI was measured at the mostly thickened portion by T2- and T1-weighted images. RESULTS: The mean mucosal thickness at the most thickened portion of paranasal sinuses, regardless of their location was 3.5mm with S.D. of 1.5mm. The mucosal thickening was observed more commonly in maxillary (79 patients, mean 3.0mm, S.D. 1.4mm) and ethmoid sinuses (80 patients,mean 2.7mm, S.D. 1.1mm) than in sphenoid (39 patients, mean 1.6mm, S.D. 1.4mm) or frontal sinuses (38 patients, mean 1.9mm, S.D. 1.4mm). CONCLUSION: Mucosal thickening of up to 6.5mm was a common finding on brain MRI of patients without evidence of sinusitis ; accuracy was 95%.