We studied the diagnostic value of computed tomographic angiography (CTA) in the posterior circulation as a noninvasive substitute for intra-arterial digital subtraction angiography (DSA). We prospectively investigated 103 patients with acute stroke in the posterior circulation. All patients underwent CTA and Doppler ultrasound, and DSA was performed in 22 patients. Vascular findings were classified in categories according to the type of lesion and the location in the vertebral, basilar, or posterior cerebral artery. In the first part of the analysis we examined the correlation between CTA and intra-arterial DSA (n=22), and in the second part that between CTA and Doppler ultrasound (n=103). Intra-arterial DSA identified 11 stenoses, 13 occlusions, and 3 hypoplasias. The correlation between DSA and CTA was best for the basilar artery (all lesions were identified with CTA). Of the 14 lesions detected in the vertebral artery by DSA 13 were also detected by CTA, but the specific type of lesion was identified in only 7 cases. The correlation between CTA and Doppler ultrasound was lower. In conclusion, CTA is thus a reliable method for detecting lesions in the posterior circulation and may replace DSA in many cases. However, particularly in the vertebral artery DSA remains the superior technique.