PURPOSE: Kawasaki disease (KD) is an acute febrile illness of infancy and early childhood characterized by diffuse vasculitis. Although the disease is generally self-limited, up to 25% of untreated patients with KD may develop coronary artery abnormalities. The acute phase of KD is characterized by marked activation of the immune system leading to increased cytokine production by immune effector cells, the induction of activation antigens on their vascular endothelium and the generation of lytic antibodies directed against vascular endothelial cells stimulated with cytokines. Immunologically, successful intravenous immunoglobulin treatment is associated with decreased lymphocyte activation, reduced cytokine secretion and the loss of cytokine induced expression of leukocyte adhesion molecules on vascular endothelium. The levels of secretory CD4 (sCD4) and secretory CD8 (sCD8) in serum correlate with T cell subset activation and may be important in monitoring and characterizing disease processes in immunological diseases. We compared serum sCD4 and sCD8 levels in acute Kawasaki disease with or without coronary artery lesion and other viral diseases such as measles and mumps. METHODS: The levels of serum sCD4 and sCD8 were measured by a sandwich enzyme immunoassay. RESULTS: 1) The level of serum sCD4 was not significantly different in patients during the acute and subacute stages of KD and viral diseases compared to control group. 2) The level of serum sCD8 was significantly elevated in patients during the acute stages of KD with coronay involvement compared to control group and continuously elevated in spite of intravenous immunoglobulin therapy. CONCLUSIONS: The results suggest that there is some activation of peripheral blood CD8+ T cells during acute KD and the activation of CD8+ T cells may contribute to the elevated levels of serum sCD8.