There has been intense interest recently among the public and the media in the possibility that increased intakes of "dietary antioxidants" may protect against chronic disease. Many research programs are underway in this area. Epidemiological evidence suggests that the consumption of fruits and vegetables may reduce the risk of both cancer and cardiovascular disease, and it has been hypothesized that this is due in part to the presence of antioxidant compounds in fruits and vegetables. As a result, these compounds have been considered together by many people and loosely termed dietary antioxidants. Closer examination, however, reveals that compounds typically grouped together as dietary antioxidants can differ quite considerably from one another, both in terms of their chemical behavior and in terms of their biological properties. This report from the Institute of Medicine's Food and Nutrition Board provides a proposed definition of dietary antioxidants so as to characterize the biological properties of these compounds.