The effects of intravenous gammaglobulin (IVGG) on changes in the peripheral blood mononuclear cell subsets during acute Kawasaki disease (KD) were studied by a random selection trial of IVGG plus Aspirin (group G) compared to Aspirin alone (group A). Group G received IVGG with 200 mg/kg per day x 5 dose. The absolute counts of peripheral blood mononuclear cell subsets were assayed by a fluorescence-activated cell sorter using monoclonal antibodies of Leu series. Before therapy, patients in each treatment group had increased counts of CD14 + macrophage/monocytes compared to healthy childhood controls (P less than 0.01). After IVGG treatment group G underwent a greater decrease in their CD14 + macrophage/monocyte counts (P less than 0.01) than group A. The changes of CD3+ T cells. Leu 7+ NK/K cells and CD19+ B cells in the peripheral blood mononuclear cell subsets with treatment in group G, were similar to those in group A. These results suggest the possibility that IVGG therapy is effective in KD by modulating macrophages/monocytes.