Inclusion Criteria:

• Age: 18 through 80 years (i.e., candidates must have had their 18th birthday, but not had their 81st birthday)
• Initiation of intravenous rt-PA within 3 hours of onset of stroke symptoms. Time of onset is defined as the last time when the subject was witnessed to be at baseline (i.e., subjects who have stroke symptoms upon awakening will be considered to have their onset at beginning of sleep)
• An NIHSSS >/= 10 at the time that intravenous rt-PA is begun

Exclusion Criteria:

• History of stroke in the past 3 months
• Previous intra-cranial hemorrhage, neoplasm, subarachnoid hemorrhage, or arteriovenous malformation
• Clinical presentation suggests a subarachnoid hemorrhage, even if initial CT scan is normal
• Hypertension at time of treatment; systolic BP > 185 or diastolic > 110 mm Hg) or aggressive measures to lower blood pressure to below these limits are needed.
• Presumed septic embolus
• Presumed pericarditis, including pericarditis after acute myocardial infarction
• Recent (within 30 days) surgery or biopsy of parenchymal organ
• Recent (within 30 days) trauma, with internal injuries or ulcerative wounds
• Recent (within 90 days) severe head trauma or head trauma with loss of consciousness
• Any active or recent (within 30 days) hemorrhage
• Known hereditary or acquired hemorrhagic diathesis, coagulation factor deficiency or oral anticoagulant therapy with INR > 1.5 or institutionally equivalent prothrombin time
• Females of childbearing potential who are known to be pregnant and/or lactating or who have positive pregnancy tests on admission
• Baseline lab values: glucose < 50mg/dl or > 400mg/dl, platelets <100,000, or Hct <25
• Subjects that require hemodialysis or peritoneal dialysis
• Subjects who have received heparin within 48 hours must have a normal partial thromboplastin time (PTT) to be eligible
• Subjects with an arterial puncture at a non-compressible site or a lumbar puncture in the previous 7 days
• Subjects with a seizure at onset of stroke
• Subjects with a pre-existing neurological or psychiatric disease that would confound the neurological or functional evaluations
• Other serious, advanced, or terminal illness
• Any other condition that the investigator feels would pose a significant hazard to the subject if Activase (Alteplase) therapy is initiated
• Current participation in another research drug treatment protocol; subject cannot start another experimental agent until after 90 days
• Informed consent is not or cannot be obtained. For example, obtunded subjects are not automatically excluded from the study. However, if the next of kin or legal guardian (i.e., the individual legally empowered in the state where the consent is obtained) cannot provide consent, randomization and entry into the study could not proceed

CT Scan Exclusion Criteria:

• High density lesion consistent with hemorrhage of any degree
• Significant mass effect with midline shift
• Large (more than 1/3 of the middle cerebral artery) regions of clear hypodensity on the baseline CT scan.

Sulcal effacement and/or loss of grey-white differentiation alone are not contraindications for treatment