Quercetin belonging to the flavonol subgroup of flavonoids is a strong antioxidant and may inhibit oxidative modification of LDL and accumulation of LDL in atherosclerotic plaques. Quercetin inhibited chemically induced tumours in animal models. To enable epidemiological studies, we determined the content of quercetin in plant foods and beverages. Tea, onions, apples and red wine were major dietary sources of quercetin; the average intake of quercetin in various countries was between 5 and 70 mg per day. Quercetin intake was inversely associated with coronary heart disease in several though not all epidemiological studies. Protection against cancer was shown in only one study. We found that quercetin-β-glycosides, the major forms of quercetin in foods, can be absorbed; conjugation with glucose enhances absorption. Pharmacokinetic studies with pure quercetin glycosides showed marked differences in absorption rate and bioavailability. We conclude that the type of glycoside is the major determinant for the pharmacokinetic behaviour of dietary quercetin.