We serially monitored cell surface antigen expression on mononuclear cells in peripheral blood isolated from patients with Kawasaki disease (KD), and found, for the first time, that a markedly increased number of CD4+CD8+ T lymphocytes was present in some of the patients (11 of the 24 cases). The cases of five of these 11 patients were complicated with coronary artery lesion (CAL); the 13 patients with normal numbers of CD4+CD8+ T lymphocytes did not have CAL. The patients' age, sex and grade of systemic inflammation evaluated by peripheral leucocyte count and serum C-reactive protein levels were not correlated to the number of CD4+CD8+ T lymphocytes. Other cell surface antigen characteristics of the CD4+CD8+ T lymphocytes included CD3+, CD45RA+, CD45RO+, CD16−, and HLA-DR+. These results indicate that the surface antigen characteristics of the KD peripheral blood examined were the same as those of Epstein–Barr virus infection without CD45RA+. These findings provide useful information for the analysis of the pathogenesis of KD.