A Mucocle (pyocele) results when the osteum of a sinus becomes permanently occluded. Continued secretion causes the cystic lesion to expand, and this expansion will destroy bone over a period of time. An infected mucocele is termed a ""pyocle"": this produces symptoms and signs of inflammation. Ethmoidal mucoceles are relatively uncommon and are usually discussed as part of the more general frontoethmoidal lesions. We were experienced a case of mucocele resolving spontaneously in the right posterior ethmoidal sinus with exophthalmos. A 45-year-old Korean male was admitted to Taegu Presbyterian Hospital, on August 22, 1980, with chief complains of exophthalmos and visual disturbance on right side. No specific gross abnormalities were disclosed on physical examination in the nasal cavity and on the simple radiographies of paranasal sinuses, except laterally producing proptosis (OD). But, C-T scan films reveal oval shaped radiolucent soft-tissue density on the medial side of the right retrobulbar area. The external ethmoidectomy approach was done.