Flavonoids are polyphenols widely distributed in the plant kingdom, and are present in fruits and vegetables regularly consumed by humans. In vitro metabolic studies of flavonoids in rat liver microsomes identified the 3',4'-dihydroxylated derivatives as the major metabolic endpoint. However, in vivo in rats almost none of this metabolite and only minor amounts of the 4'-monohydroxylated derivative was produced. Flavonoids with the 4'-monohydroxylated structure were generally not metabolised and were excreted unchanged in urine in higher amounts than other flavonoids investigated. It has for long been a controversy, whether flavonoids are absorbed as the intact glycoside or whether they have to be hydrolysed to the free aglycon prior to absorption. Recent data suggest that beta-glucosidases and maybe also lactase phlorizin hydrolase (LPH) in the small intestine are capable of hydrolysing flavonoid glucosides and these compounds are thus taken up as the free aglycon and not as the intact glycosides. LC-MS analyses of 12 dietary flavonoids in human urine showed that no flavonoid glycosides were excreted, and that the citrus flavanones and phloretin are excreted in higher amounts than the flavonols. Furthermore, total flavonoid excretion may be a useful biomarker for habitual fruit and vegetable consumption.