[en] The endoscope is a new and highly useful instrument for transphenoidal surgery (TSS), and is generally used because of its minimally invasiveness. In addition, endoscopic transsphenoidal surgey (eTSS) has a potential for more radical tumor removal at the pituitary and the parasellar regions by wider visualization and more powerful illumination. To operate these regions safely, we need to know nasal and skull base anatomy under the endoscope which looks different from images under a microscope. In this paper, we demonstrated nasal and skull base anatomy with multi-detector computed tomography, which was performed in 23 recent patients with pituitary and parasellar legions. In the nasal legion, deviation of nasal septum and deviation of sphenoid ostium are important for endonasal approach of eTSS, and often determine the difficulty of surgery in the nasal cavity. Our study showed that deviation of nasal septum was seen in 26% of patients. Deviation of sphenoid ostium was 5.5±1.5 mm from the midline. The anatomy of sphenoid sinus plays a key role in our determination of the safety of a bony opening of the sella. In addition to sellar, presellar, and concha types, carotid prominence and optic prominence are important to determine the midline orientation. Development of carotid prominence was significantly related to the extent of lateral pneumatization of sphenoid sinus (P=0.0016). Reconstructed 3D-image of sphenoid sinus was very useful in visual understanding skull base anatomy. (author)