One hundred twenty-eight patients were examined prospectively to determine the significance of mucosal thickening seen in the paranasal sinuses during routine MR imaging of the brain. On the basis of responses to a questionnaire, each patient was categorized as symptomatic (n = 60) or asymptomatic (n = 68) for paranasal sinus disease. Patients were categorized further on the basis of the maximal mucosal thickening seen by MR in any paranasal sinus. A modified t test was used to compare the prevalence of various degrees of mucosal thickening between symptomatic and asymptomatic groups. Statistically significant differences between the groups were seen only in those patients with normal sinuses and in those with 4 mm or more of mucosal thickening. We conclude that mucosal thickening of up to 3 mm is common and lacks clinical significance in asymptomatic patients. An ancillary finding is that 1- to 2-mm areas of mucosal thickening in the ethmoidal sinuses occur in 63% of asymptomatic patients. This minimal mucosal thickening in the ethmoidal sinuses is thought to be a normal variant, possibly a function of the physiologic nasal cycle.