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Aim: The azygous anterior cerebral artery (Az) is a rarely observed anomaly of the anterior cerebral artery, and its associated aneurysm is even rarer. Our aim was to evaluate 3-dimensional time-of-flight magnetic resonance angiography (3-D-TOF MRA) in the diagnosis of Az and associated aneurysms., Materials and Methods: Three thousand five hundred seventy-two consecutive patients underwent 3-D-TOF MRA at 3.0 T. Postprocessing techniques, including volume rendering (VR) and single artery highlighting, were performed by a 3-D specialist. All MRA data and clinical information were recorded and stored in a database for further analysis., Results: Fourteen patients (.39%) were identified as having an Az. Among these cases, 3 males (21.43%) had an aneurysm located at the distal bifurcation of the Az, with a mean size of 9.43 ± 3.33 mm. In MRA, the common trunk of the Az was slightly larger in diameter than the A1 segment (2.62 ± .35 mm vs. 2.54 ± .35 mm; P = .008)., Conclusions: With the VR technique, 3-D-TOF MRA is feasible and valuable in detecting an Az and associated aneurysm. Our MRA-based study has proved that the Az is a rare anomaly but has a relatively high incidence of associated aneurysms., (Copyright © 2012 by the American Society of Neuroimaging.)