Participants will be randomly assigned to a study group based on when their stroke began. Those who arrive at the hospital less than 3 hours from stroke onset will receive tPA alone or tPA with cooling (hypothermia). Those who arrive at the hospital 3 to 6 hours after stroke onset will be assigned to 1 of 4 groups—receiving either tPA alone, tPA with cooling, cooling alone, or standard medical care. Length of participation (including observation after the patient leaves the hospital) is 90 days.