Background: Hyperimmunoglobulinemia syndrome (HIES) is an inflammatory skin disease, and management of it is not satisfactory. However, high-dose intravenous gamma-globulin (IVGG) is a safe and effective treatment for Kawasaki disease. Methods: I initially treated two patients with Kawasaki disease, both of whom also had HIES, with IVGG. Subsequently, I treated patients with HIES alone, atopic dermatitis, Kawasaki disease, or idiopathic thrombocytopenic purpura with IVGG and measured serum IgE levels and in vitro spontaneous IgE production by peripheral blood mononuclear cells from patients. Direct effects of IgG on in vitro IgE production were also studied. Results: IVGG treatment remarkably improved severe eczema in patients with HIES with or without Kawasaki disease and in patients with atopic dermatitis. Moreover, serum IgE levels and in vitro spontaneous IgE production were decreased in these patients but not in patients with Kawasaki disease or idiopathic thrombocytopenic purpura. The addition of high concentrations of IgG to cultures of peripheral blood mononuclear cells from patients with HIES or atopic dermatitis decreased in vitro spontaneous IgE production. Moreover, IgG also decreased Ige production by normal B cells stimulated with interleukin-4 plus anti-CD40 monoclonal antibody. Conclusion: IVGG is effective in the treatment of severe eczema in patients with HIES and atopic dermatitis. IVGG also decreased enhanced IgE production both in vivo and in vitro.