Inclusion Criteria:

• Evidence of atrial fibrillation either on one current Electrocardiogram (ECG) or on two ECG recorded at two weeks a part during 6 months prior to study enrollment.

• Evidence of high risk of vascular events: at least one of the following risk criteria must be present:

  • are 75 years or greater;
  • on treatment for systemic hypertension;
  • prior stroke, TIA, or non-CNS systemic embolus;
  • left ventricular dysfunction with left ventricular ejection fraction (EF) estimated by echocardiogram or angiogram (radionuclide or contrast) to be < 45%;
  • peripheral vascular disease (previous peripheral artery revascularization, limb and foot amputation, or the combination of current intermittent claudication and ankle arm systolic blood pressure ratio < 0.9);
  • age 55 to 74 years; AND
  • either diabetes mellitus requiring drug therapy, or documented previous myocardial infarction, or documented coronary artery disease.

Exclusion Criteria:

• Patients will be excluded from ACTIVE if any of the following are present :

  • requirement for clopidogrel (such as recent coronary stent procedure);
  • requirement for oral anticoagulant (such as prosthetic mechanical heart valve);
  • prior intolerance to ASA or clopidogrel;
  • documented peptic ulcer disease within the previous 6 months;
  • prior intracerebral hemorrhage;
  • significant thrombocytopenia; (platelet count < 50 x 10(9)/L);
  • psychosocial reason making study participation impractical;
  • geographic reason making study participation impractical;
  • ongoing alcohol abuse;
  • mitral stenosis;
  • pregnant or nursing woman or woman of child bearing potential and not on effective birth control for at least one month prior to start of study or not willing to continue on birth control for duration of study;
  • severe comorbid condition such that the patient is not expected to survive 6 months;
  • patient currently receiving an investigational pharmacologic agent; OR
  • requirement for chronic (> 3 months) non-COX-2 inhibitor NSAID therapy.