BACKGROUND: No guidelines comprise when or to what extent sinus surgery should be done in patients with cystic fibrosis (CF) or how a CT scan of the paranasal sinuses should influence the decision. Symptoms of rhinosinusitis and/or eradication of pathogenic bacteria from the sinuses are reasons for sinus surgery. METHODS: In this observational cross sectional study, 55 CF cases had their preoperative CT scans scored according to the Lund Mackay- and the Nair-system. Correlations between the CT scans, symptoms, surgical findings and cultures obtained during sinus surgery were made. RESULTS: There was no significant correlation between the CT score and detection of pus, pathogenic bacteria or symptoms. Pus and pathogenic bacteria were found in several cases without sinus opacification on the CT scan. Non pathogenic and sterile cultures were also found in sinuses with opacification. CONCLUSIONS: A CT scan is not a valid criterion for sinus surgery in CF patients.