The architecture of ethmoid labyrinths as well as their relationships with adjacent structures are reviewed, giving special emphasis to the anatomic variants common in this area. The study was performed on six young normal subjects who were submitted to high resolution computed tomography. Approximately 25 axial scans, parallel to the floor of the anterior cranial fossa, were performed on each subject; four to five coronal CT scans were also added. Further data were supplemented by examinations of 62 patients affected by nonneoplastic disease of the paranasal sinuses. Microsurgery is replacing classic macrosurgery in the treatment of nonneoplastic disease of the paranasal sinuses. This new trend requires a perfect preoperative delineation of the anatomy. For this purpose, the present paper contains a list of 10 questions which can be answered exhaustively by computed tomography on the basis of the data illustrated. The answers cover most requirements of sinus microsurgery.