Background: This study was performed to analyze the frequencies of peripheral interferon (IFN)-γ-producing cells at the single-cell level and to determine concentrations of circulating IFN-γ in the acute and subacute phases of Kawasaki disease (KD). Methods: Ten patients with KD were studied and seven healthy children were selected as control subjects. Using immunofluorescent detection of intracellular IFN-γ in CD4-positive and CD4-negative cells, the frequencies of IFN-γ-producing cells in peripheral blood mononuclear cells were studied. Circulating IFN-γ levels were measured by enzyme-linked immunosorbent assay. Results: The frequencies of peripheral blood CD4+ and CD4– IFN-γ-producing cells in acute-phase KD patients were significantly lower than in subacute-phase KD patients and control children (p < 0.05). CD4– cells, thought to be mainly composed of CD8+ cells, appeared to be more responsible for the reduced frequencies of total IFN-γ-producing cells than CD4+ cells. There were, however, no differences in the frequencies of IFN-γ-producing cells between KD patients in the subacute phase and control children. In contrast, serum IFN-γ levels were higher in KD patients in the acute phase than in the subacute phase (p < 0.05). Conclusions: The above results show increased levels of circulating IFN-γ and decreased emergence of peripheral IFN-γ-producing cells in acute KD patients, suggesting transient infiltration of activated IFN-γ-producing cells into the inflammatory sites during acute KD. These findings also support the hypothesis that IFN-γ plays an important role in the pathogenesis of KD-related vasculitis.