Inclusion Criteria- Age 18 and older.
- Stroke or transient ischemic attack (TIA) while taking aspirin within one week of consent.
- Women of child-bearing potential will be included only if there is a negative pregnancy test and they agree to birth control during the treatment period.
Exclusion Criteria- Any medical indication for a specific antiplatelet agent (aspirin, clopidogrel, ticlopidine, aggrenox), or an indication for a combination of antiplatelet agents.
- Any medical indication for anticoagulation, except subcutaneous heparin for deep vein thrombosis prophylaxis.
- Allergy to aspirin or clopidogrel.