In most cases, the sphenoid sinus is the most posterior of the paranasal sinuses. It drains directly into the nasal cavity by an ostium in its anterior-superior medial aspect, emptying into the sphenoethmoid recess. It is located by the endoscopist by following the line from the nasal spine to the superior meatus and tracking along the posterior one-third of the superior turbinate. The ostium is variable in size and in its relationship to both the superior turbinate and the sphenoethmoid recess.