Summary Elevated IgE responses and eosinophilia observed in patients with atopic dermatitis (AD) may reflect increased responses of type 2 T-helper (Th2) cytokines with a concomitant decrease in interferon-gamma (IFN-γ) production. However, the cross-regulation of Th1/Th2 derivation and function in AD patients are incompletely characterized. Therefore, we investigated serum levels of several cytokines [interleukin (IL)-18, IL-12, IL-10, IL-2 and IFN-γ] in patients with AD to assess their possible relationships to the severity of disease. Serum IL-18 levels in AD patients were significantly higher than those in healthy controls [207 pg/mL; 95% confidence interval (CI), 172–242 pg/mL vs. 144 pg/mL; 95% CI, 116–178 pg/mL; P = 0.026]. Those IL-18 levels significantly correlated with eosinophil counts and serum soluble IL-2 receptor (sIL-2R) levels, and showed a tendency to correlate with clinical severity scores and serum IgE levels. IL-2 levels showed a significantly inverse correlation with serum IgE levels, and IL-12 levels clearly correlated with IL-10 levels. These results suggest the value of serum IL-18 levels as a parameter of AD activity and may support a possible role for IL-18 in the pathogenesis of AD. The inverse correlation between IgE levels and IL-2 levels suggests that IgE production may be inhibited by IL-2 in patients with AD. Furthermore, the correlation of IL-12 levels with IL-10 levels may support the previous reports that show the induction of IL-10 production by human natural killer cells and/or T cells stimulated with IL-12 in vitro.