PURPOSE: To illustrate frequently encountered anatomic variations on PNS computed tomography(CT) and to assess their clinical significance. MATERIALS AND METHODS: CT findings of 1523 patients, who underwent PNS CT with no history of sinus surgery, were reviewed, and prevalence of each anatomic variation and its relationship with mucociliary clearance pathway was evaluated. RESULTS: Two categories of anatomic variations were encountered. The first group included variations of the turbinates, such as concha bullosa(28.6% ), paradoxical middle turbinate(31.5%), of the uncinate process, such as medially bent or folded(36.3%), laterally bent(3.7%) or uncinate bulla(0.5%), of the ethmoidal sinus, such as Haller's ce11(28.2%), large agger nasi ce11(9.6%) or large ethmoidal bulla(23.5%) and nasal septal deviation(24. 1%), which might cause obstruction of mucociliary clearance pathway and thus give rise to secondary obstructive sinusitis. The second group included Onodi ce11(1.4%) and medial depression of the lamina papyracea (3.5%), which were not related with obstructive sinusitis. 87 cases of large agger nasi cell were associated with obstruction of nasofrontal recess. Large ethmoidal bulla(452 cases), Haller's ce11(245 cases) and true concha bullosa (25 cases) contributed to narrowing of the infundibulum. Medially bent uncinate process(220 cases), concha bullosa(157 cases), paradoxical middle turbinate(126 cases) and nasal septal deviation(93 cases) were observed in middle meatal obstruction and supreme concha (3 cases) accompanied sphenoethmoidal recess obstruction. CONCLUSION: Recognition of anatomic variations on PNS CT is important to build a treatment plan in patients with obstructive sinusitis as they may represent causes of the disease, and to avoid critical complications during functional endoscopic sinus surgery.