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Several studies reported that IL-10 levels in bronchoalveolar lavage (BAL) fluid and in the supernatant of cultured alveolar macrophages were lower in asthmatics than in healthy subjects . However, other studies have shown that levels of IL-10 messenger ribonucleic acid (mRNA) in airway tissues and its protein in BAL fluid were higher in asthmatics . A decreased level of IL-10 protein in the supernatant of peripheral blood mononuclear cells (PBMCs) from asthmatics was reported by some investigators , but not by others . Thus, the results of previous studies were inconsistent. Among those studies, the levels of IL-10 were evaluated by measuring the bulk amount of cytokine protein in the supernatant, which might not have reflected the amount of IL-10 in each type of producing cells. While alveolar macrophages are known as a major source of IL-10 , T cells recruited in the lung are the other important source. Considering their orchestrating role in allergic inflammation, the production of IL-10 in T cells needs to be evaluated. T cells circulate systemically from the affected organs to the blood stream via the lymphatic system . This unique profile enables detection of the cytokine kinetics of T cells in asthmatics using peripheral blood samples. Thus, the present study investigated the numbers of IL-10-pro-ducing, circulating T cells in atopic asthmatics, atopic nonasthmatics and healthy controls at the single cell level using cytokine flow cytometry . We hypothesized that asthmatic subjects not on regular treatment would have fewer IL-10-pro-ducing T cells than atopic nonasthmatics and normal subjects.