Three cases of basilar artery territory infarction, in which a dense basilar artery was noted, are described. The dense basilar artery was recognized on computed tomography (CT) by comparing its density with that of other unaffected intracranial vessels. This sign is thought to represent basilar thrombosis or embolism and has similarities to the dense middle cerebral artery sign. The dense basilar artery is an early sign suggesting basilar territory infarction and its use improves the CT detection of basilar territory infarction.