PURPOSE: To evaluate the usefulness of CT angiography (CTA) for the detection and assessment of intracranial aneurysms, compared with the findings of conventional angiography (CA) and surgery. MATERIALS AND METHODS: Among 243 patients who underwent CTA because of suspected intracranial a-neurysm, 116 who underwent surgery were studied. The patients were divided into two groups. Group 1 consisted of 77 patients (94 aneurysms) who underwent both preoperative CTA and CA, while group 2 comprised 39 patients (47 aneurysms) who underwent preoperative CTA only. The detection rate, size, shape and direction of the neck and its relationship to the adjacent vessel of the intracranial aneurysm seen during CTA were retrospectively compared with those seen during CA and surgery. Two radiologists worked in a blinded fashion without access to clinical information. RESULTS: In group 1, the detection rate of aneurysms seen during CTA was 89 % (84 of 94 cases), and for CA was 95 % (89 of 94 cases). Among ten undetected aneurysms, four cases were less than 3mm in size and six were out of the scanning area. In group 2, the detection rate of aneurysms seen during CTA was 94 %(44 of 47cases). Among three undetected aneurysms, one case was less than 3mm in size and two were out of the scanning area. For delineation of an aneurysm (the direction and shape of the neck, for example), CTA was equal or superior to CA in 75 of 82cases(91 % ). CONCLUSION: CTA is a useful technique for the evaluation of intracranial aneurysms in the circle of Willis, but cannot depict all vessels in the brain. CTA performed in conjunction with CA is useful for the detection of intracranial aneurysms.