Diets high in fruits and vegetables are associated primarily with a reduced risk of human cancers of the gastrointestinal tract, as well as some other common cancers including those of the lung, breast, and prostate. Plants contain hundreds of bioactive substances, including flavonoids, which are a large group of compounds consisting of two phenolic benzene rings linked to a heterocyclic pyran or pyrone. This report reviews published epidemiologic studies that have examined associations of flavonoid intake with cancer risk in humans. Findings to date are extremely limited, but there is relatively consistent evidence from these studies that flavonoids, especially quercetin, reduce risk of lung cancer. Evidence is modest for other epithelial cancers. Further research is needed to confirm these findings before public health recommendations about flavonoids can be offered to consumers.