Researchers are reporting that the reactivity of platelets—cells involved in forming blood clots—may be a useful marker in the future for predicting heart disease risk in apparently healthy people and may provide an important target for prevention. In the largest long‐term study of platelet reactivity effects on cardiovascular outcomes, researchers showed that so-called intrinsic platelet hyperreactivity (specifically to ADP) is associated with increased risk of future heart attacks and strokes. ADP (adenosine diphosphate) is a substance that stimulates platelet aggregation. The study uses population data from the Framingham Heart Study. Funded by NHLBI, the study appears in the Journal of the American Heart Association.