Relatively little is known about the prevalence of sinusitis in a general population. Patients with suspected intracranial neurological disease can be considered as representative of the general population. On magnetic resonance imaging nearly 60% of the sinuses were affected. If the most common lesion (a maxillary polyp or cyst) was excluded, still 40% of the sinuses were affected. Above all the maxillary sinuses are involved with 40% abnormal images followed by the anterior ethmoidal sinuses with 14%. The other sinuses are less likely to be affected with 2.5% of the sphenoidal, 2% of the frontal and 1.5% of the posterior ethmoidal sinuses.