In the ASCET study, 1000 patients with documented coronary heart disease will be randomized to either continued treatment with aspirin 160 mg/d or change to clopidogrel 75mg/d. Clinical endpoints will be recorded for at least 2 years and related to the initial aspirin response, assessed by the PFA-100® method, to investigate whether aspirin non-responders have higher composite event rate than responders or whether Clopidogrel treatment in patients non-responsive to aspirin will reduce their risk of future clinical events. The clinical events are the composite of unstable angina, myocardial infarction, stroke or death.