Total IgE, RAST results with tree pollen allergens, and prick test results with birch, grass and mugwort pollen allergens were correlated to 872 hay fever patients' reported food hypersensitivity (FH). A positive correlation was found between FH and the RAST and prick test results with birch pollen allergen. At each level of birch pollen sensitivity the incidence of FH was lower in patients with high total IgE than in those with lower total IgE. A negative correlation was found between grass pollen allergy and FH in birch pollen allergics. It is suggested that antigens in some foods have a specific ability to bridge anti-birch IgE molecules on mast cells. An explanation of the negative correlation between FH and total IgE and grass pollen allergy could be that a high number of non-birch-specific IgE molecules on the mast cells will reduce the probability that two anti-birch IgE molecules should bind on nearby sites.