IL-13 plays a crucial role in the development of allergic asthma by several mechanisms, including induction of IgE antibodies, airway eosinophilia and hyper-reactivity. We previously established a deregulated production of IL-13 by T cells from allergic asthma patients. In this report we describe the identification of a novel IL-13 promoter polymorphism (C to T exchange) at position −1055. The IL-13 −1055 TT genotype is associated with allergic asthma (P = 0.002), altered regulation of IL-13 production (P Ͻ 0.002), and increased binding of nuclear proteins to this region. We postulate that the presence of this polymorphism predisposes to the development of allergic asthma.