BACKGROUND Fetal posterior cerebral artery occlusion is rare and often presents with severe neurological symptoms. Although acute recanalization therapy is commonly used for cerebral vessel occlusion, unruptured cerebral aneurysms can be hidden distal to the occluded vessels. OBSERVATIONS An 87-year-old man presented with consciousness disturbance and right hemiparesis. The authors diagnosed left fetal posterior cerebral artery occlusion and performed mechanical thrombectomy. A stent retriever was deployed from the middle cerebral artery M1 segment across the mural thrombus of the internal carotid artery. After the first pass, the fetal posterior cerebral artery remained occluded, with confirmation of a contrast effect around the thrombus. Because the anatomical course of the fetal posterior cerebral artery was unidentified, the procedure was stopped. At 1-week recovery, magnetic resonance imaging revealed complete recanalization and a fetal posterior cerebral artery aneurysm hidden within the occluded site. Blood flow was directed to the aneurysm, and the thrombus within the aneurysm simultaneously occluded the fetal posterior cerebral artery. LESSONS To avoid critical complications following mechanical thrombectomy for fetal posterior cerebral artery occlusion, hidden aneurysms should be suspected when a “fried egg–like” contrast effect is observed around the thrombus.

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