Interleukin 27 (IL-27) acts as a versatile cytokine in the early regulation of Th1 initiation and in the negative regulation of the Th2 factor GATA-3. IL-27, which was discovered as a novel heterodimeric cytokine of the IL-12 family, consists of two subunits, the Epstein-Barr virus-induced gene 3 (EBI3) and p28. The IL-27 cytokine is mediated by one of the receptor chains (WSX-1) of the IL-27 receptor that is highly expressed on CD4(+) T lymphocytes and NK cells. Although signaling of IL-27/WSX-1 interactions have been recognized in the down-regulation of airway hyper-reactivity and in lung inflammation during the development of allergic asthma, little is known about the role of single nucleotide polymorphisms (SNPs) of IL-27 and individual susceptibility to asthma. To address this question, we have examined the five exons and the boundary intron sequences of IL-27P28, including the promoter regions, with the aim of identifying sites of variation that may be useful for understanding the genetic influences of this gene. We identified four SNPs, g.-964A > G, g.2905T > G, g.4603G > A and g.4730T > C, and analyzed the genotype and allele frequencies between asthma patients and healthy controls. Our results strongly suggest that the g.-964A > G polymorphism of IL-27p28 is most likely associated with susceptibility to asthma. Moreover, we elucidate the haplotype frequencies of g.2905T > G, g.4603G > A and g.4730T > C in terms of their relative correlation with asthma patients and healthy controls.