PURPOSE: To determine the anatomic variations that can lead to optic nerve damage during the sugical treatment of posterior paranasal sinus lesions MATERIALS AND METHODS: two hundred optic nerves of 100 persons were examined using ostiomeatal unit CT (OMU CT). The anatomical features of this nerve and posterior paranasal sinuses were classified into four types : the optic nerve adjacent to the sphenoid sinus without indentation of the sinus wall (type 1); the optic nerve adjacent to the sphenoid sinus, causing indentation of the sinus wall (type 2); the optic nerve passing through the sphenoid sinus (type 3); and the optic nerve adjacent to the sphenoid sinus and posterior ethmoid sinus (type 4). Bony dehiscence around the optic nerve and pneumatization of the anterior clinoid process were also evaluated. RESULTS: The anatomical classification of the optic nerve and posterior paranasal sinuses was as follows : type 1, 1326 (66%); type 2, 60 (30%); type 3, 6 (3%), and type 4, 2 (1%). Bony dehiscence around the optic nerve had developed in 58 cases (29%) and pneumatization of the anterior clinoid process in 13 (6.5%). These conditions were most common in type 3 optic nerve, and second most common in type 2. CONCLUSION: The 2 and 3 optic nerve, bony dehiscence around the optic nerve and pneumatization of the anterior clinoid process are the anatomic variations that can lead to optic nerve damage during the surgical treatment of posterior paranasal sinus lesions. To prevent optic nerve damage, these factors should be carefully evaluated by OMU CT.