Flavonoids are polyphenolic compounds that occur ubiquitously in fruits and vegetables. Over 4,000 different flavonoids have been described, and they are categorized into flavonols, flavones, catechins, flavanones, anthocyanidins and isoflavonoids. Flavonoids have many different biological effects in numerous mammalian cell systems, showed both in vitro and 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. In epidemiological studies the intake of flavonoids was inversely associated with subsequent coronary heart disease and cancer. Soy, St. John's Wort, Silybum marianum, Ginkgo biloba, Citrus species, Vaccinum mirtillus, Hawthorn and tea are medicinal plants containing flavonoids whose efficacy in the treatment of a variety of diseases has been demonstrated in numerous clinical studies. The main question that has not been answered satisfactorily deal with the triad absorption/metabolism/efficacy of flavonoids and is actually an important unsolved problem in judging their many alleged health effects. The aim of this review is based on providing by means of experimental and clinical studies information about efficacy and toxicity of flavonoids, especially focusing on infantile leukaemia for which there has been an international warning without sufficient and affordable scientific data.