Background: Type I allergic symptoms in the oropharyngeal mucosa upon contact with plant-derived food in patients with pollen allergies have been termed oral allergy syndrome (OAS). IgE cross-reactivity between pollen and food allergens represents the molecular basis for this phenomenon. The sensitizing allergen source (pollen or plant food) in OAS is a controversial issue. Objective: We sought to determine the primary sensitizing molecules in patients with OAS. Methods: We used recombinant birch pollen (rBet v 1 and rBet v 2) and plant food allergens (apple, rMal d 1; celery, rApi g 1; and carrot, rDau c 1), as well as natural pollen (birch and timothy grass) and plant food (apple, peach, kiwi, hazelnut, celery, and carrot) allergens, to identify cross-reactive allergens by using qualitative immunoblot inhibitions. In addition, we determined the percentage of plant food–specific IgE that can be preadsorbed with recombinant and natural pollen allergens by quantitative RAST inhibitions by using sera from 71 patients with OAS. Results: Preincubation of sera with recombinant and natural pollen allergens led to an almost complete inhibition of IgE binding to plant food allergens in Western blots, as well as in RAST inhibition experiments. In contrast, recombinant plant food allergens poorly inhibited IgE binding to Bet v 1. Conclusion: Most IgE epitopes in plant food recognized by patients with OAS are resembled by pollen allergens. Thus pollen allergens may be responsible for the elicitation and maintenance of OAS. (J Allergy Clin Immunol 2000;105:116-25.)