Immediate hypersensitive reactions (IHR) induced by buckwheat ingestion are considered to be IgE-mediated. However we found 28 subjects without IHR to buckwheat ingestion out of 46 subjects who had positive RAST values for both buckwheat and rice antigens. The IHR-positive group showed significantly higher RAST values for buckwheat antigens (p < 0.01) but lower RAST values for rice antigens (p < 0.01) than did the IHR-negative group. RAST values for buckwheat and rice were significantly correlated with each other (p < 0.01) in the IHR-negative group, but not in the IHR-positive group. An effective dose-dependent inhibition was obtained in a RAST inhibition assay between homologous combinations of inhibitor and disc antigens such as rice and rice or buckwheat and buckwheat. The IHR-positive group showed no significant RAST inhibition between heterogeneous combinations of rice and buckwheat antigens. In contrast, the IHR-negative group showed a significant decrease in IgE binding even in the RAST inhibition assay between heterogenous combinations. These results led to the conclusion that there is cross-reactivity with IgE antibodies between buckwheat and rice and that IgE antibodies from IHR-negative subjects might recognize the epitopes on buckwheat antigens which cross react with rice antigens, whereas IgE antibodies from IHR-positive subjects might bind to buckwheat-specific epitopes.