Aim:To evaluate the clinical and radiological features, treatment modalities, and outcomes of unusual aneurysms located beyond the origin of the major branches in the posterior circulation, and to introduce changes in the recent treatment trends due to rapid innovations in endovascular technology.Material and Methods:This was a retrospective study of patients who underwent treatment for these unusual aneurysms, including those that were identified in regular follow-up after treatment, between March 2009 and April 2023. Medical information including the radiological features of the aneurysms, incidences of rebleeding, associated vascular diseases, treatment modalities, and outcomes, was documented.Results:A total of 22 cases consisting of two unruptured and 20 ruptured aneurysms were included. Their locations were the posterior cerebral artery in four cases, the superior cerebellar artery in three, the anterior inferior cerebellar artery in two, and the posterior inferior cerebellar artery in 13. Sixteen were saccular, five fusiform, and one blister-like. Eight were pseudo-aneurysms and pre- or intra-operative rebleeding occurred in 13 (65%) of 20 cases with ruptured aneurysms. Five aneurysms coexisted with causative vascular diseases such as arteriovenous malformation, moyamoya disease, or dolichoectasia. Four cases were treated by microsurgical operations and 18 by endovascular operations. In one of the microsurgical cases and five of the endovascular cases, parent arteries were sacrificed. Stents were used in six cases, including low-profile stents in four. Intermediate guiding catheters were used in seven cases for distal access. Full recoveries were seen in 17 cases and death occurred in three.Conclusion:Treatments for these aneurysms are technically demanding due to the high rate of rebleeding, difficult accessibility, and inevitable necessity of sacrifice of the parent artery in some cases. However, advancing endovascular techniques and devices enable distal access to the lesion and help preserve the parent artery.