We investigated the changes of CD5+ B cells in the peripheral blood of 20 Kawasaki disease (KD) patients. The percentage of CD5+ B cells in the total lymphocytes and in the total B cells significantly decreased during the acute phase of KD(p< 0.01), compared to that in the age-matched normal control subjects. After intravenous immunoglobulin(IVIG) treatment, the percentage of CD5+ B cells increased, but was still lower than that in the normal controls(p< 0.01). During the convalescent phase of the disease, the percentage of CD5+ B cells was restored to the normal levels. The levels of CD5+ B cell percentage in the total B cells of the patients with acute febrile disease showed similar levels to age-matched normal controls. The decreased CD5+ B cells in the patients with KD provides an additional abnormal immunological finding during the acute phase of the disease.