Although osteoma of the paranasal sinus is comparatively common, involvement of the sphenoidal sinus region is extremely rare. Lame reported that a total of twelve cases had been described in the European literature since 1800. Mikaelin, Kulczynski and Dolan, since then, described three cases. This paper deals with two surgical cases of sphenoidal sinus osteoma, reviewing literature on paranasal sinus osteoma and differential diagnosis in the sphenoidal sinus lesion. Case 1. A 20-year-old man was referred to us by an ophthalmologist, complaining of left visual disturbance (visual acuity: 0.02). He had a past history of chronic sinusitis. Neurological examination revealed papilledema on the left. Plain skull X-ray film showed marked destruction of the sphenoidal sinus and tomography demonstrated thinning and expanding of the sella floor with cloudiness in the sphenoidal sinus. CT scan showed an irregular high density mass in the sphenoidal sinus. Sublabial-nasoseptal sphenoidotomy was performed. The sinus was occupied by homogeneous osseous tissue surrounded by thin fibrous tissue. The osseous part was removed as much as possible by a high speed air drill. Histopathologic examination confirmed mature osteoma. Postoperative course was uneventful and left visual acuity was improved up to 0.06. He is now doing well without recurrence for six months. Case 2. A 59-year-old woman was admitted to our hospital, complaining of acromegaly for ten years. She had a past history of chronic sinusitis, hypertension, diabetes mellitus and left putaminal hemorrhage which was evacuated five months before. She had typical acromegalic features and serum levels of growth hormone was 65.8 ng/ml.(ABSTRACT TRUNCATED AT 250 WORDS)