We estimated volume changes in the posterior bony wall of the sphenoid sinus, as well as alterations in nasal function (including olfactory function and subjective symptoms), after sphenoid mucosal repositioning using the endoscopic endonasal transsphenoidal approach (EETSA). During 2010 and 2021, 13 patients underwent sphenoid mucosal repositioning during EETSA, while 24 patients (the control group) did not. Pre- and postoperative paranasal sinus computed tomography and the Mimics program were used to evaluate three-dimensional changes in the posterior wall of the sphenoid sinus. All patients underwent the Connecticut Chemosensory Clinical Research Center (CCCRC) test, the Cross-Cultural Smell Identification Test (CCSIT), Nasal Obstruction Symptoms Evaluation (NOSE), the Sino-Nasal Outcome Test-20 (SNOT-20), and visual analog scale (VAS) evaluation. The increase in the volume of the posterior wall of the sphenoid sinus after surgery was objectively smaller in the sphenoid mucosal repositioning group than in the control group (P=.046). However, this did not affect olfactory function (as revealed by the CCCRC test or the CCSIT) or subjective symptoms (as revealed by the NOSE, SNOT-20, and VAS scores) (all P > .05). Surgical closure via sphenoid mucosal repositioning during EETSA reduces the volume of the posterior wall of the sphenoid sinus and facilitates re-operation. We suggest that sphenoid mucosal repositioning is appropriate during EETSA. 4 Laryngoscope, 132:965-972, 2022.