Recent research on inflammatory sinus disease has implicated a central role for the ethmoid labyrinth, which may influence changes in the maxillary and frontal sinuses. CT can provide excellent definition of the paranasal sinuses and particularly the ethmoids, which is a prerequisite for endoscopic surgery. We describe a prospective series of 110 coronal CT scans performed on patients with a clinical diagnosis of sinusitis who had undergone diagnostic nasal endoscopies and medical treatment prior to surgery. 86% of the scans showed abnormal mucosal thickening. The ethmoids were affected in 73% and the maxillary sinus in 64%. Pneumatization of the middle turbinate was a common variant and when present was associated with anterior ethmoid disease in 60% of patients. Anterior ethmoid inflammatory changes were demonstrable in 95% of patients with maxillary sinus disease. Direct coronal CT can readily demonstrate disease in the infundibulum, frontal recess and posterior ethmoids in the same orientation confronting the endoscopist, and helps in the planning of ethmoidal surgery. Following surgery the sinuses can be directly inspected in outpatients which reduces the need for any further plain radiographs. It is important that the diagnosis of sinusitis is not based on CT findings alone as isolated areas of mucosal thickening are common in the normal population.