Background: The bioavailability of natural food folates is lower than that of synthetic folic acid, but no agreement exists as to the extent of the difference. Objective: In a 4-wk dietary intervention study, we determined the aggregate bioavailability of food folates from fruit, vegetables, and liver relative to that of folic acid. Design: Seventy-two healthy adults were randomly divided into 4 treatment groups. Group A (n29) received a high-folate diet with 369gfoodfolate/dandaplacebocapsule;groupsB,C,andD(n 14or15)receivedalow-folatedietwith73gfoodfolate/dandfolic acidcapsules.Thesecapsulescontained92gfolicacid/dforgroup B, 191 g for group C, and 289 g for group D. In addition, all 72 subjects daily ingested a capsule with 58 g( 13C11)-labeled folic acid. We measured the percentage of (13C11)-labeled folate in plasma folate at the end of the intervention and ascertained the changes in serum folate concentrations over the 4 wk of the inter- vention. Results: Bioavailability of food folate relative to that of folic acid was 78% (95% CI: 48%, 108%) according to (13C11)-labeled folate and 85% (52%, 118%) according to changes in serum folate con- centrations.