We examined linkage between markers at and near the IL4 gene and atopic dermatitis (AD) in 88 Japanese nuclear families. Affected sib pair analysis suggested linkage between the IL4 gene and AD (SPLINK lod=2.28). Transmission disequilibrium testing showed a significantly preferential transmission to AD offspring of the T allele of the -590C/T polymorphism of the IL4 gene (p=0.001). A case-control comparison suggested a genotypic association of the TT genotype with AD (odds ratio=1.88, p=0.01). Since the T allele was reported to be associated with increased IL4 gene promoter activity compared with the C allele, our data indicate that genetic differences in transcriptional activity of the IL4 gene influence AD predisposition, particularly in Japanese, because of a high frequency of the T allele.