Background: Acute multiple brain infarction (AMBI) pattern on diffusion-weighted imaging (DWI) is associated with arterial and cardiac sources of embolism. The DWI characteristics of patients with stroke due to vertebrobasilar arterial dissection and atherosclerotic disease have not been reported in detail. Abstract:Objective: To describe the DWI stroke patterns in patients with posterior circulation occlusive disease to determine mechanisms of ischemia. Design: Retrospective analysis of infarct patterns in patients with symptomatic vertebrobasilar disease. Setting: Large community-based teaching hospital. Patients: Patients admitted with stroke due to vertebrobasilar disease were identified retrospectively.Patients were included if DWI was obtained within 7 days of symptom onset. Main Outcome Measure: Infarct patterns were analyzed according to established templates of vascular territories. Results: Eleven patients with vertebral dissection and 39 patients with atherothrombosis were identified.An AMBI pattern was present in 8 (72%) of 11 patients with arterial dissections and 25 (64%) of 39 patients with atherosclerotic disease (P = .48). Distal embolism to the terminal branches of the basilar artery occurred with equal frequency in both groups and was found in half of all cases. Isolated thalamic infarction did not occur. Pontine infarction was noted in 2 (18%) of 11 patients with dissections and 18 (46%) of 39 patients with atherosclerosis (P = .09). Cerebellar border zone involvement was found in 14 (36%) of 39 patients with atherosclerosis and 4 (37%) of 11 patients with dissections (P = .6). Conclusions: Large arterial disease is frequently associated with AMBI in the posterior circulation. The incidence of AMBI was comparable to that reported in the anterior circulation. This DWI study supports the importance of embolism as the main mechanism of infarction in patients with vertebrobasilar occlusive disease. On the basis of our experience, large-vessel vertebrobasilar disease rarely causes isolated small-vessel thalamic infarction. The DWI characteristics of patients with stroke due to vertebrobasilar arterial dissection and atherosclerotic disease have not been reported in detail. Design: Retrospective analysis of infarct patterns in patients with symptomatic vertebrobasilar disease. Patients: Patients admitted Results: Eleven patients with vertebral dissection and 39 patients, with stroke due to vertebrobasilar disease were identified retrospectively. Patients were included as if DWI was obtained within 7 days of symptom onset. Main Outcome Measure: Infarct patterns were analyzed according to established templates of vascular territories. with atherothrombosis were i The AMBI pattern was present in 8 (72%) of 11 patients with arterial dissections and 25 (64%) of 39 patients with atherosclerotic disease (P = .48). Distal embolism to the terminal branches of the basilar artery occurred with equal frequency in both groups and was found in half of all cases. Isolated thalamic infarction did not occur. Pontine infarction was noted in 2 (18%) of 11 patients with dissections and 18 (46%) of 39 patients with atherosclerosis (P = .09). Cerebellar border zone involvement was found in 14 (36%) of 39 patients with atherosclerosis and 4 (37%) of 11 patients with dissections (P = .6). Conclusions: Large arterial disease is frequently associated with AMBI in The posterior circulation. The incidence of AMBI was comparable to that reported in the anterior circulation. This DWI study supports the importance of embolism as the main mechanism of infarction in patients with vertebrobasilar occlusive disease. On the basis of our experience, large-vessel vertebrobasilar disease rarely causes isolated small-vessel thalamic infarction.