Functional endoscopic sinus surgery is widely used for treatment of inflammatory disease of paranasal sinus. The surgery should be performed on the basis of clinical judgement and sinus CT to determine the extent of disease and structual anomalies. And pre-operative awareness of dehiscence of lamina papyracea is helpful to avoid orbital or other complications on the surgery. CT scans were taken for 276 patients(202 males, 74 females) with suspicious chronic sinus inflammation and there were analysis for dehiscence of lamina papyracea. The results were obtained as follows : 1) The total incidence of dehiscence of lamina papyracea was 10.9%, and the natural dehiscence of primary anatomic variation was 5.8%, the acquired dehiscence was 5.1%, no demonstrable difference of sex or age. 2) Of 30 patients with dehiscence of lamina papyracea, 43.3% had dehiscence on right side, 50.0% had dehiscence on left side, and 6.7% had dehiscence on both sides. So the majority of patients had unilateral dehiscence. 3) Regarding to degree of dehiscence, less then 50% were found in 87.5% of patients. 4) In natural dehiscence, all cases were found on the anterior ethmoid sinus and all cases accompanied orbital fat herniation. 5) Locations of dehiscence were anterior ethmoid sinuses in 84.4%, posterior ethmoid sinus alone in no patient, and anterior & posterior ethmoid sinuses of 15.6% at the same time. 6) In patients with dehiscence of lamina papyracea, the incidence of medial deviation of orbital muscle was 6.7%.