Sphenoid sin us mucocele is a rare entity (1% to 2% of paranasal sinus mucoceles) and difficult to diagnose. Due to the numerous important anatomical structures nearby, it can cause a variety of symptoms, among which headache and visual abnormalities are the most important. In July 2002, we treated a 64-year- old female. Her symptoms were right hemicrania and episodic postural dizziness. The brain CT scan showed brain atrophy and atherosclerosis of the bilateral internal carotid and basilar arteries. She was subsequently admitted to our Neurology ward. However, the next day she showed exacerbated symptoms with right sided diplopia and ptosis with blurred vision. An emergent MRI was performed showing a huge homogenous sphenoid sinus mass, which was expanding outside the sphenoid sinus putting pressure on the pituitary gland and the cavernous sinus. The Neurology Department then consulted our Department and we immediately performed endoscopic surgery. In surgery, the anterior wall of the sphenoid sinus was removed and mars upialization was performed, whereby much yellowish mucopus was drained. The symptoms of headache and visual abnormalities disappeared ten days after surgery and no evidence of recurrence has been noted since.