The interleukin-12B (IL12B) gene encodes the p40 chain of IL-12, a pro-inflammatory cytokine that antagonizes Th2 phenotype and, hence, may have a critical role in the pathogenesis of allergic asthma. In this report, we describe the identification of a novel IL12B promoter polymorphism (T-to-A exchange) at position -536. The IL12B -536AA genotype was significantly associated with asthma in the Chinese populations (P=0.011, odds ratio=2.227). The risk-associated A allele was linked to reduced expression of IL12B mRNA levels and IL12B production in asthmatic patients. Luciferase reporter assay provided evidence that risk-associated A allele reduced the promoter activity of IL12B gene compared with those of the promoter containing the protective T allele. We further observed that decreasing binding effects between the risk alleles A of IL12B and CCAAT/enhancer binding protein alpha (C/EBPα) through A allele sequence mediated streptavidin-conjugated agarose pulldown and biotin-labelled A allele mediated electrophoretic mobility shift assay. We also observed additive effects of the risk alleles of IL12B and decreased mRNA levels of C/EBPα in asthmatic patients. Therefore, we postulated that the presence of -536A allele in IL12B promoter could predispose to the development of allergic asthma.