Study to Assess the Safety and Effectiveness of the Penumbra System

Inclusion Criteria: Clinical signs consistent with acute ischemic stroke 18 to 79 years of age Neurological deficit resulting in an NIH Stroke Scale (NIHSS) score > 8 TIMI 0 or TIMI I flow in vessels accessible to the Penumbra System Signed informed consent Present within 8 hours of stroke symptom onset; ineligible or refractory to intravenous tissue plasminogen activator (t-PA) therapy if presenting within 3 hours of symptom onset. Exclusion Criteria: Evidence of rapidly improving neurological signs of stroke at time of enrollment NIHSS > 30 or coma Females who are pregnant Vessel tortuosity too difficult to allow endovascular access Known hemorrhagic diathesis, coagulation deficiency, or on oral anticoagulant therapy with an International Normalized Ratio (INR) > 3.0 Partial thromboplastin time (PTT) greater than 2 times the lab normal Admission platelets < 30,000 Pre-existing neurological or psychiatric disease that could confound the study results Known severe allergy to contrast media Uncontrolled hypertension Computed tomography (CT) evidence of significant mass effect with a midline shift CT reveals evidence of large hypodensity region > 1/3 of the middle cerebral artery territory CT reveals evidence of intracranial hemorrhage CT reveals significant mass effect with midline shift Angiographic evidence of an arterial stenosis proximal to the occlusion that could prevent thrombus removal Angiographic evidence of preexisting arterial injury Life expectancy of less than 90 days Participation in another clinical investigation that could confound the evaluation of the study device.