A 51-year-old woman complained of intermittent nasal obstruction and rhinorrhea. Sinus roentgenograms and polytomograms revealed a small mucocele of the right sphenoid sinus. There was no encroachment of the orbital apex or intracranial contents. The mucocele was marsupialized via an external ethmoidectomy, and the scant tissue present inside the sinus was examined histologically. The pathologic diagnosis was hemangiopericytoma. The small tumor had apparently originated at the sinus ostium, thus creating the mucocele. Several months postoperatively, a small, vascular tumor nodule was noted in the incision. No recurrence was evident intranasally. A course of 5,040 rads of external beam radiation therapy was given to the right ethmoid labyrinth and right sphenoid. Since completing her radiation therapy, there has been no recurrence. This case is of interest because the tumor occurred as a mucocele of the sphenoid sinus.