An atopic girl experienced an anaphylactic reaction after eating cookies containing pecan nuts. Investigations revealed that she had developed IgE antibodies exclusively specific against allergenic determinants present in aged or heated pecan nuts, but not in fresh pecans. These neoallergenic determinants were located on protein(s) with a molecular weight of approximately 15 kDa. Neoallergens appearing during heating or storing of foods may be important in some anaphylactic reactions.