Background: Atopic dermatitis (AD) and Japanese cedar pollinosis (JCP) are common chronically allergic diseases associated with the activation of T-helper 2 cells. Recent studies have shown that polymorphisms in the genes for IL-4 receptor α chain (IL4RA) may contribute to susceptibility of AD and JCP, although the differences in the involvements of loci of IL4RA gene between AD and JCP are unclear. In this study, we investigated the role of polymorphisms in IL-4RA gene in conferring susceptibility to the development of AD and/or JCP using a family analysis and an association analysis in a Japanese population. Methods: Five polymorphisms in the IL-4RA gene, C-3223 T, T-1914C, T-890C, Ile50Val and Glu375Ala, have been genotyped using PCR-based methods in 75 trios families, including 15 AD families, 30 JCP families, and 30 families with combination of AD and JCP in the family analysis. Forty-five AD, 60 JCP and 125 control children constituted the association study. Results: The transmission disequilibrium test showed that the allele of Ile50 was significantly transmitted to children with JCP alone (p<0.05). Haplotype analysis showed that the -3223 T/Ile50 haplotype was preferentially transmitted to both AD (p<0.01) and JCP children (p<0.01), while that the C-3223/Ile50 haplotype was preferentially transmitted to only JCP children (p<0.01). The association study showed that -3223 T and haplotype of -3223 T/Ile50 were associated with aD children, but not with JCP. Ile50 was associated with both AD and JCP. Conclusions: Our data suggest that -3223 T and the -3223 T/Ile50 haplotype were risk factors for AD. Ile50 allele seems to be involved in both JCP and AD. Interactions of the IL-4RA loci may play a role both conferring susceptibility and modulating severity of AD.