OBJECTIVE This report examines the usefulness of the preoperative image to orient the surgeon in the sphenoid sinus during endoscopic endonasal transsphenoidal surgery (ETSS). METHODS ETSS was performed in 100 cases of sellar lesion and used to classify the sphenoid sinus septum shape. Preoperative computed tomography and magnetic resonance imaging were performed for 2 types of coronal imaging: conventional and oblique. Expected sphenoid sinus septum shape was compared with those from ETSS to estimate concordance. The confirmation rate of anatomic landmarks in the sphenoid sinus by endoscopic observation was compared in various types of septum and the identification rate in oblique coronal imaging was also examined. RESULTS The most common septum shape was single type (31%), followed by branched (26%), parallel (18%), none (12%), cross (9%), and bridge (4%) types. In oblique coronal images, preoperative evaluation and endoscopic findings were consistent in 93%-100% of cases. However, with conventional coronal images, the concordance rate was 22.2%-83.9%, and in the none, branched, and cross types, the concordance rate was significantly lower than that for oblique coronal images. Although confirmation of the midline through estimation of landmarks by endoscopic observation was difficult in 33 cases, preoperative computed tomography and magnetic resonance imaging showed landmarks in all cases and oblique coronal images best indicated the midline. CONCLUSIONS Use of oblique coronal images in addition to conventional images provided good orientation of anatomic structures in the sphenoid sinus. The combination of preoperative imaging and endoscopic observation could allow safer surgery in ETSS.