Noncontrast computed tomographic scans (CT scans) may show a hyperdense basilar artery before a brainstem infarct is visualized. This early sign should assist clinicians in confirming the diagnosis of basilar artery thrombosis. In a review of admission records of 750 patients with acute cerebrovascular disease from July 1991 to June 1993, at Saint Louis University Hospital, 20 patients were identified with clinical signs of nonlacunar, vertebrobasilar distribution infarction. Eight of these had pontomesencephalic ischemia. Their neuroimaging studies and medical records were evaluated. Four patients with acute clinical signs of pontomesencephalic infarction were found to have a hyperdense basilar artery on CT scans. The scans of 2 patients were excluded because of dolichoectasia; in the other 2 patients, the basilar artery appeared normal on the CT scan. The hyperdense basilar artery was detected within the early hours of neurological symptoms and often was the only detectable abnormality on the scan. In 3 patients extensive brainstem infarcts subsequently developed and they died. Basilar artery thrombosis was confirmed by pathological study in all these patients. In the fourth patient basilar artery occlusion and a large pontine infarct were evident by magnetic resonance imaging and angiography. A hyperdense basilar artery is a common feature on CT scans of patients presenting with an early clinical diagnosis of thrombosis. Untreated, the hyperintense basilar artery often portends a poor prognosis. Its ready recognition should guide further interventional studies and treatment.