We asked whether in atopic dermatitis (AD) increased T cell apoptosis in staphylococcal enterotoxin B (SEB)-activated cultures of peripheral blood mononuclear cells (PBMCs) is characteristic of the exacerbation of the disease or connected with skin colonization by Staphylococcus aureus. The clinical status of the patients was evaluated using the SCORAD index. The number of bacteria colonizing patients' skin lesions was determined by the cfu method. Mononuclear cells isolated from peripheral blood were stimulated by SEB and the apoptosis of CD3+ cells in culture was determined by flow cytometry using the monoclonal antibody APO2.7. The cytokine production in the culture supernatants was determined by ELISA and Cytometric Bead Array kits. T cell apoptosis was increased, while the production of interferon (IFN)-? was reduced in cultures of PBMCs of AD patients during exacerbation. The proportion of CD3+APO2.7+ cells positively correlated with the density of S. aureus recovered from skin lesions, but not with SCORAD index. By contrast, SCORAD index, but not S. aureus density, negatively correlated with IFN-? production. Furthermore it was found that the presence of S. aureus on uninvolved skin distinguishes a group of severe cases with high serum IgE level, increased T cell apoptosis, and reduced production of tumor necrosis factor ? in SEB- -stimulated cultures. Among AD patients the increased activation-induced T cell apoptosis observed in SEB- -stimulated cultures is related to skin colonization by S. aureus. The presence of bacteria on uninvolved skin is a feature of a distinct group of AD patients.