Sphenoid sinus mucoceles are exceedingly rare. Most have occurred in men between the ages of 30 and 40 years. We report a case of sphenoid sinus mucocele that was unusual not only for the age of the patient (13 yr), but for the massive size of the lesion (59 x 53 mm). The patient presented with a 20-day history of unilateral proptosis, decreased vision, diplopia, and photophobia. He also complained of severe bilateral nasal obstruction, nasal stuffiness, and a mucoid nasal discharge. Endoscopic decompression of the left sphenoid sinus was performed, and approximately 200 ml of thick, sterile mucoid secretion was aspirated. Despite the size of the mucocele, no significant destruction of the sphenoid walls was evident. Postoperatively, the patient's symptoms improved significantly.