Purpose: To establish whether the clinical finding of pituitary adenomas´ localization above sphenoid sinus septa is an anecdotal observation or a regular occurence. – – Material and Methods: – Sphenoid sinus septum anatomy was initially studied in 100 consecutive CT exams of the paranasal sinuses. The highly variable anatomy of the septa was classified in three groups: No septum (type I), one medial septum (type II), multiple septa (type III). Next, in pre-operative MR studies of 43 patients with histologically confirmed pituitary microadenoma (? 10 mm), the position of the adenoma relative to the sphenoid sinus septa was analyzed. – – Results: – In 33 of the 43 cases (77%), the adenoma was localized directly above the insertion of a sphenoid sinus septum in the floor of the sella. Type III septum anatomy was by far most common. – – Discussion: – Sphenoid sinus septum anatomy is highly variable. The fact that pituitary microadenomas are located above the insertion of a septum in nearly four fifths of our cases makes this occurence so typical that we named it ´´sphenoid sinus septum sign´´. – Neither anatomy, histology of the pituitary gland and pituitary adenomas, or embryologic development of the sellar region can offer an explanation for this finding.