血栓化動脈瘤は壁が硬くそのままでは頸部クリッピングが不可能なことが多い. 小型の場合にはクリップを複数組み合わせることによって対応できることもあるが, 大型の場合には内部の血栓除去を行い血管形成的にクリップをかけることで, ようやく動脈瘤の処理が可能となる. また, 動脈瘤に癒着した脳や動脈穿通枝などを剥離するためにも血栓を摘除し可動性を持たせる必要がある. しかしながら血栓の摘除には動脈瘤の切開が必要で, 出血のコントロールや親動脈遮断による虚血の危険性に留意しなければならない. われわれが最近経験した血栓化動脈瘤の2例を提示し, バイパスやMEPの意義と手術の留意点について検討する. 症例 <症例1>患者は68歳, 女性, 既往歴に特記すべきことはない. 平成14年11月頃より左眼の視力低下を自覚していた. 12月4日後頸部痛があり, 徐々に歩行障害と呂律難が進行し近位を受診し, CTでくも膜下出血と左中大脳動脈の血栓化巨大動脈瘤を認め, 12月13日当院に紹介入院となる.We report 2 cases with giant/large thrombosed aneurysm. One patient, who presented with subarachnoid hemorrhage, had a giant thrombosed aneurysm of the middle cerebral artery and underwent surgery. To prepare for the parent artery occlusion, a STA-MCA bypass was set and motor evoked potential (MEP) was monitored. The aneurysm was cut open, the thrombus was removed, and clipping was performed. Another patient, who presented with visual disturbance, had a large thrombosed aneurysm of anterior communicating artery, which occupied the tubercullum sellae. The aneurysm was clipped after removal of the thrombus. Since the wall of a thrombosed aneurysm is too hard to clip, the thrombus must be removed before clipping. To prevent premature rupture, a thrombus should be carefully removed, paying attention to the nature of the thrombus. In preparation for parent artery occlusion, installation of bypass and MEP monitoring is effective. Even after setting a bypass, the blood flow to the perforating arteries should be carefully monitored with MEP.