We investigated 45 patients with Kawasaki disease (KD) and report the first simultaneous determination of tumor necrosis factor (TNF), interleukin 2 receptor (IL-2R) and interferon-gamma (IFN-gamma) in the serum during acute phase. Serum levels of TNF were measured by a sandwich enzyme-linked immunosorbent assay. Serum levels of soluble IL-2R and IFN-gamma were measured by a sandwich enzyme immunoassay and radioimmunoassay, respectively. Serum levels of TNF, IL-2R, and IFN-gamma were seen to increase during the acute phase of KD. In KD patients with coronary-artery lesions (CAL), the percentage of positive cases for TNF (greater than or equal to 10 U/ml), IL-2R (greater than or equal to 1056 U/ml), and IFN-gamma (greater than or equal to 0.3 U/ml) was higher than that in patients without CAL. Our results suggest that aggressive activation of immunocompetent cells develops in KD with CAL.