PURPOSE: To determine whether CT angiography can predict the parent artery of an anterior communicating aneurysm on the basis of characteristics of the aneurysm and precommunicating anterior cerebral artery. MATERIALS AND METHODS: Eighteen patients with anterior communicating aneurysm were studied prospectively using CT angiography and conventional angiography. The parent artery of an aneurysm was predicted by evaluating aneurysm neck location, direction of aneurysm projection, and size of the precommunicating anterior cerebral artery, as seen on CT angiography. A somatom Plus-4 spiral CT scanner was used and shaded-surface display three-dimensional images were constructed. RESULTS: Aneurysm neck was located near the parent artery in 12 cases(66.7%), and aneurysm projection was against the parent artery in 11 cases(61.1 %). The parent artery of the anterior cerebral artery was larger in 15 cases (8 3 .3 %), including ten cases of hypoplasia or agenesis of the contralateral anterior cerebral artery. In 17 of 18 aneurysms(94.4%) the parent artery seen on DSA was correctly predicted by CT angiography. CONCLUSION: Teh parent artery of an anterior communicating aneurysm can be predicted by evaluating a-neurysm neck location, direction of aneurysm projection, and precommunicating anterior cerebral artery, as seen on CT angiography.