The anatomy of the sphenoid sinus, as it relates to endoscopic sinus surgery, was studied in 93 cadaver heads (186 sphenoid sinuses) using endoscopic dissections as well as sagittal sections. The relationship of the sphenoid sinuses to the carotid artery, optic nerve, floor of sella turcica, as well as other important structures, were verified and discussed. The recesses of the sinus as well as its ostium and accessory septa and crests were described and their clinical importance was discussed. Pertinent measurements were included wherever appropriate.