A large mucocele of the frontal and ethmoidal sinuses with extracranial, intracranial, and intraorbital extension accompanied by total destruction of the left frontal sinus was treated successfully. We chose a two-stage surgical procedure for radical removal of the lesion. It is important to ascertain the degree of dural involvement, not only because extensive involvement of the dura mater presents technical surgical difficulties, but also because this knowledge provides clues regarding the pathology of the lesion. Even after surgical treatment, the mucocele may recur if the patent sinus ostium is exposed to the dura mater, or if the ostium subsequently becomes blocked.