Bilateral watershed (WS) infarction at the borderzone of the middle cerebral artery territory are typically related to severe hypotensive events. Unilateral WS infarctions have sometimes been reported in occlusion of the ipsilateral internal carotid artery (ICA), but their pathogenesis and prognostic implications are poorly known. Twenty-six of 154 (17%) consecutive patients with ICA occlusion had a computed tomography-proved ipsilateral WS infarct. Severe active heart disease with hypotension and syncope (p less than 0.0001), severe contralateral ICA disease (p less than 0.001), and elevated venous hematocrit values (p less than 0.001) related to heavy smoking were more frequent in the patients with WS infarction than in other patients. Delayed infarctions in the territory of the main branches of the middle cerebral artery distal to the ICA occlusion correlated with a visible stump or emboligenic changes (ulcerated or irregular stenosis) on the collateral channels, but this was not true for delayed WS infarctions. These findings suggest that most of the WS infarctions were hemodynamic. The patients with WS infarct had a higher death rate (9.9% per year) than did the patients without WS infarct (2.3% per year), suggesting that heart disease should be particularly closely managed in patients with WS infarctions. As WS infarctions were the most frequent type of infarction distal to an occluded ICA, appropriate treatment of all potential causes of systemic hemodynamic disturbances may be crucial in the long-term management of patients with ICA occlusion.