Apple allergy, as well as allergies to nuts, stone fruits and other fruits and vegetables, is very common in patients with hay fever caused by birch pollen. The observed clustering of hypersensitivities is due to cross‐reactions of allergen‐specific IgE antibodies. In the case of apple allergy, patients frequently report that the symptoms are more severe after ingestion of mature fruits. We therefore investigated whether changes of IgE binding properties arose during the development of Boskoop and Golden Delicious apples. Using one tree for each strain, extracts were prepared from fruits at different states of maturity. Sera of patients allergic to birch pollen and apples were studied by means of non‐competitive and competitive ELISA (EAST and EAST inhibition) as well as by immunoblotting followed by absorption/reflection densitometry. The results of the three methods agreed closely and revealed that the allergenic potency increased strongly during maturation of Golden Delicious apples and to a lesser degree during ripening of Boskoop apples. The extract with the highest allergenic activity was obtained from mature Golden Delicious. The differences in the potency of IgE binding were due to the occurrence of an 18 kD allergen in Golden Delicious apples, whereas only very low amounts of this protein were detected even in extracts of mature Boskoop.