To compare the clinical value of three-dimensional (3D)-CT angiography using spiral scanning and 3D-time-of-flight (TOF) MR angiography in the diagnosis of cerebral aneurysms, the results of these studies were compared in 12 patients with a total of 17 aneurysms. In seven of the 17 aneurysms, anatomic details were visualized better by 3D-CT angiography than by MR angiography. Both studies demonstrated five aneurysms equally well. MR angiography delineated three aneurysms better than 3D-CT angiography. Both studies failed to demonstrate two very small aneurysms. In 3D-CT angiography, aneurysms were viewed from many view angles, allowing the assessment of the spatial relationship between an aneurysm and the surrounding vascular and bone structures. Considering the use of contrast material and irradiation, 3D-CT angiography is of value in delineating aneurysms which are equivocal on MR angiography. In addition, 3D-CT angiography is a potential preoperative adjunct to conventional angiography.