The mechanism(s) responsible for increased IgE synthesis in atopic dermatitis (AD) are unknown, but they may be related to either decreased interferon gamma (IFN-gamma) and/or increased interleukin (IL)-4 production. In this study we examined peripheral blood mononuclear cells (PBMCs) from 21 patients with AD, six patients with psoriasis, and 22 nonatopic healthy controls for IFN-gamma and IL-4 production after stimulation with concanavalin A (Con A). The Con A-induced proliferative response of AD PBMCs was similar to the response of healthy controls (p = 0.9). After mitogen stimulation, however, AD culture supernatants contained significantly less IFN-gamma (p = 0.001) but increased IL-4 (p = 0.001) compared with supernatants from nonatopic controls. In contrast, PBMCs from patients with psoriasis produced normal levels of IFN-gamma and IL-4 in vitro. Since IL-4 is known to decrease IFN-gamma synthesis, we examined the effect of neutralizing anti-IL-4 on IFN-gamma production. Anti-IL-4 significantly increased IFN-gamma production in patients with AD (p = 0.008) and nonatopic controls (p = 0.02) but did not normalize IFN-gamma production by AD PBMCs. Supernatants from AD PBMCs, but not supernatants from nonatopic PBMCs, induced IgE synthesis in PBMCs from nonatopic donors (p = 0.02). When an anti-IFN-gamma receptor antibody, which blocks cellular binding of IFN-gamma, was added to supernatants from nonatopic controls their capacity to induce IgE synthesis was significantly greater (p = 0.03). These results demonstrate an imbalance of IL-4 and IFN-gamma production, which may contribute to increased IgE synthesis in AD.