Atopic dermatitis (AD) is frequently associated with eosinophilia, highly elevated immunoglobulin E (IgE) levels and increased levels of T-helper 2-type (Th2) cytokines in skin lesions due to infiltrating T cells. Interleukin-12 (IL-12), in combination with interferon-gamma (IFN-gamma), inhibits IgE synthesis and Th2 cell function. As the IFN-gamma-inducing cytokines IL-12 and IL-23 utilize IL-12Rbeta1 as part of their receptors, it is possible that polymorphic variants of the IL-12Rbeta1 (IL12RB1) gene might determine an individual's susceptibility to AD. Here, we carried out a systemic search for genetic variants of the human IL12RB1 in Japanese subjects and identified 48 genetic variants. In a case-control association study, we found that promoter polymorphisms -111A/T and -2C/T were significantly associated with an increased risk of AD under a recessive model. The -111T-allele frequency in the independent population of child asthmatics was also much higher than that in the control group. In addition, the -111T/T genotype was progressively more common in AD with high total serum IgE levels in an IgE-level-dependent manner. Deletion analysis of the IL12RB1 promoter suggested that the -265 to -104 region that contained the -111A/T polymorphic site harbored an important regulatory element. Furthermore, we showed that the -111A/T substitution appeared to cause decreased gene transcriptional activity such that cells from -111A/A individuals exhibited higher IL12RB1 mRNA levels than those from -111T allele carriers. Our results suggested that in individuals with the -111T/T genotype, reduced IL-12Rbeta1 expression may lead to increased Th2 cytokine production in the skin and contribute to the development of AD and other subsequent allergic diseases.