Peripheral blood mononuclear cells (PBMCs) obtained from atopic dermatitis (AD) patients produced low levels of IFN-γ in response to Dermatophagoides farinae antigen (Der f Ag) plus IL-2 or OKT3 MoAb in contrast with PBMCs obtained from healthy donors. The reduced IFN-γ production in AD patients’ T cells appeared to be derived from the defect of CD4+ T cells but not CD8+ T cells. Indeed, from the cytoplasmic staining analysis of cytokines, it was demonstrated that the frequency of IFN-γ producing CD4+ T cells (TH1 cells) in AD patients was markedly lower than that of healthy donors. From the phenotypic analysis using flow cytometry, it was also found that the number of CD4+ CD45RO+ memory type T cells was significantly reduced in AD patients compared with that of healthy donors. In addition to quantitative defect of memory type CD4+ T cells, functional defect of CD4+ CD45RO+ memory type T cells was also demonstrated in AD patients. Enriched CD4+ CD45RO+ T cells obtained from AD patients, who exhibited greatly reduced delayed-type hypersensitibity (DTH) response in tuberculin test, showed no significant TH1 immunity in terms of IFN-γ production by stimulation with OKT3 MoAb or purified protein derivative (PPD). Thus, the immunological abnortmality of TH1 immunity in AD patients appeared to be induced in concomitant with both the quantitative and qualitative defect of memory type CD4+ T cells.