We describe our technique and results of dynamic, reconstructed, three-dimensional (3-D) computed tomographic (CT) angiography in our first 18 patients with either complex aneurysms, producing subarachnoid hemorrhage or the compression of neighboring structures, or small asymptomatic aneurysms, and assess the role of this technique in treatment planning. A dynamic, infused CT scan producing 1.5-mm cuts of the area of interest was performed, and the two-dimensional images were reconstructed in three dimensions with the ISG Allegro system (ISG Technologies, Toronto, Ontario, Canada). Results were compared with intra-arterial digital subtraction angiography and magnetic resonance angiography. All aneurysms over 3 mm and half of the aneurysms less than or equal to 3 mm were demonstrated by 3-D CT angiography. The neck of the aneurysm, its relationship to the parent artery and associated branches, and its relationship to surrounding bony structures, such as the anterior clinoid, were also demonstrated. This was especially useful for ophthalmic aneurysms. The 3-D image could be rotated on the screen to mimic the operative approach. The active, filling portion and the thrombosed part of partially thrombosed, giant aneurysms could be seen on the same image, and the active, filling portion could be better appreciated with 3-D CT angiography than with magnetic resonance angiography. A partially treated, acrylic-coated aneurysm that could not be completely assessed otherwise has been demonstrated by this technique. 3-D CT angiography is useful in the evaluation of aneurysms, especially when the local anatomy is obscured or distorted by its mass or by the anterior clinoid.(ABSTRACT TRUNCATED AT 250 WORDS)