The prevalence of allergic diseases such as asthma, allergic rhinitis and atopic dermatitis has increased all over the world during the last two decades. Dietary change is supposed to be associated with this increase. If it is the case, an appropriate intake of foods or drinks with anti-allergic functions is expected to stop the increase. Flavonoids, ubiquitously present in vegetables, fruits or teas possess anti-allergic activities. Flavonoids inhibit histamine release, synthesis of IL-4 and IL-13 and CD40 ligand expression by basophils and mast cells. Analyses of structure-activity relationships of representative flavones showed that luteolin, apigenin and fisetin were the strongest inhibitors of IL-4 production with an IC₅₀ value of 2–5 μM and determined a fundamental structure for the inhibitory activity. Quercetin and kaempferol showed a substantial activity with an IC₅₀ value of 15–18 μM. The inhibitory activity of flavonoids on IL-4 and CD40 ligand expression were thought to be mediated through their inhibitory action on activation of nuclear factor of activated T cells and AP-1. Administration of flavonoids into atopic dermatitis-prone mice prevented the onset of dermatitis and serum IgE elevation and ameliorated the severity of dermatitis even after the onset. In addition a preliminary trial of flavonoids for adult patients with atopic dermatitis showed a significant effect. Recent epidemiological studies reported that a low incidence of asthma was significantly observed by population with a high intake of flavonoids. Thus, these evidences will be helpful for the development of low molecular compounds for allergic diseases and it is expected that an appropriate daily intake of flavonoids may be an effective complementary and alternative medicine and a preventative strategy for allergic diseases.