Flavonoids are polyphenolic compounds that occur ubiquitously in foods of plant origin. Over 4000 different flavonoids have been described. Flavonoids have a variety of biological effects in numerous mammalian cell systems, in vitro as well as in vivo. Recently much attention has been paid to their antioxidant properties and to their inhibitory role in various stages of tumour development in animal studies. Hitherto, analytical research on flavonoids was mainly aimed at identification and not at quantification. As a consequence, no data were available for epidemiological investigations of flavonoid intake and chronic diseases. Flavonoids in foods are mostly linked to sugars, the so-called glycosides. As one parent compound or aglycone, e.g. quercetin, may be linked to a number of different sugars, quantification in foods is complex. Hydrolysis of the glycosides and subsequent determination of the parent aglycones simplifies this task. Following this approach we developed and validated an HPLC method and determined the flavonol and flavone content of vegetables, fruits and beverages commonly consumed. Subsequent epidemiological evaluation showed that the intake of flavonols and flavones was inversely associated with coronary heart disease in both a prospective cohort study and in a cross-cultural study. However, no relation with cancer risk could be established.