SummaryBackground Urticaria is the commonest cutaneous reaction caused by aspirin or other nonsteroidal anti-inflammatory drugs. The pathogenesis of aspirin-induced urticaria (AIU) is not fully understood, but appears to involve mast cell activation and neutrophil infiltration. Objectives To investigate the genetic contribution of interleukin (IL)-18, which can amplify acute inflammation by promoting mast cell activation, neutrophil migration and cytokine production, to the pathogenesis of AIU. Methods A case-control association study was performed using 275 patients with AIU and 196 normal healthy controls in a Korean population. Two promoter polymorphisms of the IL18 gene ()607A ⁄C and )137G ⁄C) were genotyped using the primer extension method. The functional effect of the IL18 gene promoter polymorphism was investigated through in vitro studies including a luciferase reporter assay and electrophoretic mobility shift assays (EMSAs) and ex vivo studies involving neutrophil chemotaxis assays. Results A significant association was detected between both AIU in general and the aspirin-intolerant acute urticaria (AIAU) phenotype and the IL18 promoter polymorphism )607A ⁄C. Patients with AIAU showed higher frequencies of the C )607 G )137 haplotype, ht1 [CG], compared with controls (P = 0AE02). Moreover, ht1 [CG] showed a high transcript haplotype by the luciferase activity assay, and EMSAs identified a )607C allele-specific DNA-binding protein as CREB2. Neutrophil chemotactic activity was highest in subjects with AIU exhibiting the high transcript haplotype, ht1 [CG] (P = 0AE019). Conclusions The high transcript haplotype ht1 [CG] of the IL18 gene may contribute to the development of acute cutaneous inflammation sensitive to aspirin, leading to the clinical presentation of AIAU.