The first case of a primary mucocele of the petrous apex is presented and the differential diagnosis is briefly discussed. Since this area is unavailable for direct examination, a thorough radiographic evaluation is essential. A mucocele should be suspected when a lytic lesion has a multiloculated appearance and when the contralateral petrous apex is highly pneumatized. Even then, a biopsy may still be needed to make a definitive diagnosis. The appropriate treatment for these cystic lesions is fistulization into a radical mastoid cavity or an exteriorized sphenoid sinus.