Quercetin, a dietary antioxidant flavonoid, has anticarcinogenic properties. We quantified the absorption of quercetin in ileostomists. Absorption was 52 ± 5␏or quercetin glucosides from onions, 17 ± 15␏or quercetin rutinoside, and 24 ± 9␏or quercetin aglycone. The plasma quercetin concentration in subjects with an intact colon, after ingestion of fried onions, apples and pure quercetin rutinoside, decreased slowly with elimination half-lives of about 25 h. Thus, repeated dietary intake of quercetin will lead to accumulation in plasma. The relative bioavailability of quercetin from apples and rutinoside was onethird of that from onions. Absorption kinetics and bioavailibility might be determined by the type of glycoside. Dietary quercetin could increase the antioxidant capacity of blood plasma.