Heart attacks and strokes are often caused by blood clots that clog arteries. Blood is made up of a liquid (serum) in which many different particles (formed elements) are suspended. Formed elements include such things as red cells, white cells, and platelets. Clot formation depends largely on activities of the platelets. Platelets can respond to external signals from other parts of the body by changing their surface characteristics, such as their ability to stick to each other, forming a clot. Doctors have found that they can prevent platelets from becoming sticky by giving certain types of medication, including aspirin and clopidogrel. Researchers have recently shown that when both of these drugs are given together, they are more effective than aspirin alone in preventing heart attack, stroke, or cardiovascular death in high-risk patients. The trouble is that clopidogrel is very expensive and it is not certain that the cost of the combination can be justified.