Serum IL-10 level in Kawasaki disease(KD) was tested. In the KD patients' sera during the acute phase, the levels of IL-10 were markedly elevated (122.0 +/- 39.1 pg/ml) compared to 3.7 +/- 1.7 pg/ml in the control subjects (p < 0.001). The serum IL-10 levels remained elevated in the subacute phase (16. 7 +/- 9.7 pg/ml, p < 0.001) and were restored to the normal levels(7.9 +/- 3.9 pg/ml) during the convalescent phase. In the patients with acute febrile disease, the serum IL-10 level increased significantly (34.4 +/- 14.1 pg/ml, p < 0.001) compared to that of the age-matched control subjects, but were not as high as in acute phase of KD(p < 0.005). This increase in serum IL-10 levels in KD may contribute to the up-regulation of humoral immunity and to the down-regulation of acute inflammation due to an increase in proinflammatory cytokines.