A total of 120 participants from approximately 30 different medical centers will be enrolled into this study. Participants will be randomized to either receive treatment by mechanical embolectomy with the Merci Retriever or Penumbra System and standard medical care or treatment with standard medical care alone. Standard care for stroke patients may include intravenous fluid, careful regulation of blood pressure, blood-thinning medicine (such as heparin or warfarin), anti-platelet medicine (such as aspirin or clopidogrel), and rehabilitation therapies.