STUDY DESIGN A retrospective analysis using prospectively collected data from 3-dimensional computed tomography angiography (3D-CTA). OBJECTIVE To investigate the frequency of anomalous vertebral arteries (VA) and variations of the posterior portion of the circle of Willis (PPCW) using 3D-CTA for preventing perioperative iatrogenic vascular complications. SUMMARY OF BACKGROUND DATA Some studies have reported that preoperative 3D-CTA is useful for determining the VA blood flow in the cervical spine. However, preoperative 3D-CTA has not been used for evaluating PPCW, which functions as vessels collateral to the basilar artery in the case of iatrogenic VA injury. METHODS The study included 100 consecutive patients (61 males and 39 females; mean age, 60.4 ± 15.4 yr; range, 11-86 yr) who underwent cervical decompression and/or instrumentation between April 2008 and May 2012. We measured the diameters of the VA (VAD), posterior communicating artery (PCOMD), first segment of the posterior cerebral artery (P1D), and basilar artery (BAD) twice and determined the frequency of anomalous VA and PPCW variations. RESULTS Hypoplastic VA, hypoplastic PCOM, and hypoplastic P1 were detected in 11 (11.0%), 81 (81.0%), and 13 patients (13.0%), respectively. Hypoplastic PCOM-P1 and hypoplastic basilar artery were observed in 87 (87.0%) and 3 patients (3.0%), respectively. Overall, 47 patients (47.0%) possessed some degree of abnormal VA blood flow.There were 7 patients (7.0%) with both unilaterally hypoplastic VA and bilaterally hypoplastic PCOM-P1s, in whom iatrogenic VA injury on the dominant side could have caused lethal vascular complications. We termed the hypoplastic VA of the contralateral side without collateral vessels as "critical VA." CONCLUSION The VAs and PPCW vary considerably. Preoperative 3D-CTA provides important information for preventing tragic vascular complications caused by iatrogenic VA injury. Taking the risk of radiation into consideration, we recommend this method for patients at the highest risk for iatrogenic VA injury.