OBJECTIVE: Computed tomographic angiography (CTA) is useful for rapid and relatively noninvasive detection of aneurysms in the circle of Willis. The purpose of this study is to compare CTA with conventional cerebral angiography (CCA) and to assess usefulness of CTA in detection of the anatomic delination of intracranial aneurysms of the circle of Willis in subarachnoid hemorrhage. PATEINTS AND METHODS: 126 consecutive patients with known SAH or suspected intracranial saccular aneurysms underwent CTA and CCA from January 2002 to June 2003. Using surface shaded display post-processing technique, CTA was interpreted for the presence, location and anatomic features of the aneurysms and also the image obtained with CTA was compared with CCA image. RESULTS: In 112 operated patients, 125 aneurysms were detected. CCA revealed 123 cerebral aneurysms and CTA revealed 113 aneurysms. Two of the 125 cerebral aneurysms were located outside of the imaging volume of CTA, 10 cases were not detected and false positive were 2 cases. The sensitivity of CTA was 90.3%. The results obtained with CTA comparing with that of CCA were almostly equal in detection of aneurysm location and delineation of aneurysmal neck. However, CTA provided a 3-dimensional representation of aneurysm and also it was very useful for surgical planning. CONCLUSION: CTA can be a diagnostic tool for the patients with acute subarachnoid hemorrge due to a ruptured aneurysm of the circle of Willis and provides adequate anatomic detail for surgical planning especially to complex cerebral aneurysms. However, it is obvious that CCA is necessary because of the limitations of CTA including its difficulty in detecting unusually located aneurysms including those in cavernous sinus or distal artery, and acquiring dynamic flow information.