The purposes of this study were to determine the prevalence of concha bullosa in patients with chronic sinusitis, to assess the origin of the pneumatisation (from the anterior or posterior ethmoid cells), and to evaluate the significance of the concha bullosa in the genesis of inflammatory sinus disease. We reviewed the CT studies of 308 patients with chronic sinusitis, assigning four grades of pneumatisation: absent, small, medium and large. Unilateral or bilateral concha bullosa was found in 164 patients (53%). In 79% of cases it was pneumatised via the posterior ethmoidal cells and in 21% via the anterior. A small concha bullosa was associated with abnormalities of the maxillary sinus, ethmoidal cells and ostiomeatal unit respectively in 49%, 28% and 34% of cases, whereas with a large concha bullosa the association was 55%, 36% and 41% respectively on the ipsilateral side and 55%, 32% and 41% on the contralateral side. The usually accepted hypothesis that the concha bullosa may contribute to the pathogenesis of inflammatory sinus disease seems doubtful.