Background Intravenous immunoglobulin therapy, widely used for various autoimmune and systemic inflammatory diseases including Kawasaki disease (KD), is occasionally associated with thromboembolic adverse effects caused by an abrupt increase in blood viscosity. Scarce information is available, however, regarding the effect of single high-dose immunoglobulin therapy for KD on blood viscosity. Methods and Results Eleven boys and 5 girls (mean age: 2.1 years) with acute-phase KD underwent single high-dose immunoglobulin therapy. Plasma viscosity before the treatment was 1.18 centipoises (SD =0.06), but it significantly rose to 1.34 centipoises (SD =0.06) (p<0.001). Multiple regression analysis revealed that, among various factors including hematocrit, plasma concentrations of total protein, immunoglobulin G (IgG), immunoglobulin A (IgA), and immunoglobulin M (IgM), only plasma IgG concentration was included in the model to explain plasma viscosity (R² =0.59, p<0.001). Conclusions Single high-dose regimen for acute-phase KD increases blood viscosity and therefore might increase the risk of thromboembolism. (Circ J 2005; 69: 962 - 964)