Morphological changes in the paranasal sinuses are regularly noted on MRI, but little is known about the incidence and significance of these changes in the general population. The purpose of this study was 1) to classify the morphological changes in the paranasal sinuses seen on MRI 2) to investigate the prevalence, site and type of paranasal abnormalities and 3) to evaluate the significance of the findings by relating them to the presence of sinusitis symptoms, allergy, smoking habits and seasonal variations. In a one-year period, 404 patients referred to MRI for suspected intracranial neurological pathology were prospectively investigated. Before undergoing the scan the patients completed a questionnaire. The observed morphological conditions were classified so that mucous thickening < 5 mm was recorded as normal; > or = 5 mm, total sinus opacification or fluid and polyps as pathological. According to this classification 31.7% of the patients had pathological findings in the sinuses. A significantly higher incidence was found in the winter period and in patients with symptoms associated to sinusitis. "Blocked nose" was the only symptom occurring significantly more often in patients with pathological changes. There was no significant relationship between paranasal sinus abnormalities and sex, age, allergy, smoking habits, previous events of sinusitis or frequent events of colds. Criteria for pathological MRI findings in the paranasal sinuses are desirable and might improve the basis for a decision on the correct medical or surgical treatment.