The defect in interferon-gamma (IFN-gamma) production that results in a T helper cell type 2-dominated response may be responsible for a decrease in the apoptosis of allergen-activated T cells in asthma. We investigated the effect of recombinant IFN-gamma on proliferation, Fas/Fas ligand (FasL) expression, and apoptosis in allergen-stimulated peripheral blood mononuclear cells obtained from atopic, asthmatic patients and nonatopic, control subjects. The addition of IFN-gamma at the start of cultures markedly inhibited the proliferative response to a specific allergen in cells from all asthmatic patients, whereas no change was observed in cells from nonatopic, control subjects. IFN-gamma induced an increase in the expression of Fas and FasL by allergen-stimulated CD4+ T cells from asthmatic patients and caused the apoptosis of these cells. A Fas-blocking monoclonal antibody prevented the inhibitory effect of IFN-gamma on allergen-induced proliferation. These results suggest that IFN-gamma inhibits the proliferation of allergen-stimulated CD4+ T cells from atopic, asthmatic patients by inducing the surface expression of Fas and FasL, which in turn triggers their apoptotic program. The defect in IFN-gamma production involved in the allergic, immune response may therefore be responsible for a decrease in apoptosis of allergen-activated T lymphocytes in the airways of atopic, asthmatic patients.