Susceptibility to atopic diseases is known to involve genetic factors. The interleukin-4 (IL-4) receptor- alpha gene (IL4R) reportedly is involved in the development of atopy. A recent study has shown the Ile50 allele of a polymorphism (Ile50Val) of IL4R to be associated with atopy. The objective of this study was to replicate this association and confirm the possible role of the Ile50Val polymorphism of IL4R in the etiology of atopic asthma in a Japanese population. We conducted a transmission disequilibrium test in 86 families identified through asthmatic children. A case-control study was also carried out using both atopic and control subjects. The IL4R Ile50Val polymorphism was genotyped by a PCR-restriction fragment length polymorphism method using an intronic upstream primer. The IL4R Ile50 allele was not preferentially transmitted to atopy- or to asthma-affected children. Neither the Ile50 allele nor the Ile50/Ile50 genotype was more prevalent in the atopic subjects than in the control subjects. Our findings indicate that the Ile50Val polymorphism of IL4R does not play a substantial role in genetic predisposition for the etiology of atopy or asthma in this Japanese population.