Because house dust mite (HDM)-specific IgE antibody (IgE-RAST) is usually not detectable in infants with atopic dermatitis (AD), HDM has not been regarded as the cause of infantile AD. The level of HDM-specific lymphocyte proliferation (expressed as stimulation index measured by flow cytometry [SIF]), however, was found to be markedly elevated in AD infants. This suggests that the sensitization of T cells to HDM extract occurs even in infancy. Moreover, the level of HDM-SIF is correlated closely with the severity of infantile AD, suggesting that HDM is a major cause of this disease not only in adults and children but also in infants. Although the level of HDM-SIF did not correlate with the level of HDM-specific IgE-RAST in infants with AD, it did intimately correlate with the absolute number of peripheral blood eosinophils. Because T lymphocytes are known to secrete some cytokines, such as IL-5, that enhance the proliferation of eosinophils, we measured the IL-5 production by peripheral blood mononuclear cells in infants with AD on stimulation with HDM extract. The amount of IL-5 production is significantly higher in infants with AD, as well as in children with AD, than that found in nonatopic control subjects. Moreover, the level of IL-5 production is correlated closely with the level of HDM-SIF in infants with AD. Taken together, these results suggest that HDMs play an important role in the development of infantile AD by inducing IL-5 production from HDM-specific T lymphocytes.