Platelet activation is an important pre-thrombotic event. The elucidation of its pathophysiology could contribute to a reduction in the mortality associated with coronary artery disease-the foremost cause of death in the UK. We examined the platelets of 27 patients with angiographically documented coronary artery disease. All patients had stable angina and were taking their regular medication-including aspirin. We demonstrated significantly increased expression of GP53 and activated GPIIb/IIIa on the platelet surface using a sensitive flow cytometric method of detection. Comparison was made with a control group of 35 patients. Seventeen of the patients had coronary angioplasty carried out. Serial studies of these patients demonstrate an immediate and sustained increase in platelet activation and this has important implications for prevention of restenosis after angioplasty.