Arterial lesions, mechanisms, territory, and clinical features of anterior inferior cerebellar artery (AICA) territory infarcts are only based on necropsy cases. To our knowledge, no large clinical series has been reported. We selected nine consecutive patients with AICA territory infarction confirmed by magnetic resonance imaging and angiography. Atherosclerosis was the only cause and all patients were hypertensive. Patients with pure AICA territory infarcts (n = 4) were diabetic and likely had basilar branch occlusion due to basilar artery plaques that extended into the AICA or microatheroma that blocked the AICA origin. These patients had no or had only recently had (1 day) prodromata. Patients with AICA plus infarct (n = 5) had basilar artery occlusion at the AICA and reconstitution of the distal basilar artery by collaterals through hemispheric anastomoses from the posterior inferior cerebellar arteries and posterior communicating arteries. All these patients except one had prodromata. In seven of nine patients, cranial nerve involvement indicated a lateral pontine lesion in the territory supplied by the AICA. Only two patients had the complete AICA syndrome, and none of the patients had isolated vertigo. The outcome was good in seven of nine patients. Isolated unilateral AICA infarcts should be regarded as most likely due to small artery atherosclerotic disease in diabetic patients. More widespread infarctions that include that AICA territory are due to basilar artery occlusive disease.