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Introduction: The primitive persistent trigeminal artery (PTA) is the most common carotid-basilar anastomosis, but because of its extremely low incidence, it is poorly understood. The purpose of this study is to clarify the features of PTA and its variants (PTAV) based on a large magnetic resonance angiography (MRA) series. Methods: MRA was performed on continuous 16,415 patients (8,610 males and 7,805 females) between October 2005 and September 2008 using two 1.5-T systems and one 1.0-T system. These all MRAs were evaluated by neuroradiologists. Results: The incidence of both PTA and PTAV was 0.68% (48 cases of PTA, 50 cases of PTAV, and five unclassified). Among them, 65 cases originated from the left internal carotid artery and 38 from the right. There were 44 cases of lateral type PTA and four cases of medial type. Hypoplasia of the basilar artery proximal to the union of PTA was recognized as follows: no hypoplasia in 12 cases, moderate hypoplasia in 22 cases, and severe hypoplasia in 13 cases. Distance from SCA and the union was an average of 6.7 mm. Four cases of pre- and postoperative cerebral aneurysms were recognized in 103 cases (3.9%). Conclusion: We have identified details of both PTA/PTAV. The incidence of PTA was similar to previous studies, and the co-existence of cerebral aneurysm was also similar when compared to patients in the general population without PTA or PTAV. [ABSTRACT FROM AUTHOR]