To investigate the class II major histocompatibility antigen expression on coronary arterial endothelium of Kawasaki disease and immunophenotypes of the infiltrating cells in the coronary vascular lesions, myocardial sections from a patient who died during the acute stage of Kawasaki disease were studied using an immunoperoxidase technique. The mononuclear cells in the lesions mainly consisted of macrophages and T cells, whereas B cells and NK/K cells were not seen. The majority of T cells reacted with Leu-3a antibodies, and only a few reacted with Leu-2a antibodies. Cells bearing the interleukin-2 receptor, indicative of activated T cells, were also found in the lesions. To determine the distribution of class II antigen, we used anti-HLA-DR antibodies. The massive expression of HLA-DR antigen on mononuclear cells was found in the lesions. In addition, the HLA-DR activation antigen was expressed on the coronary arterial endothelium at the infiltrates in which macrophages and T cells coexisted. In contrast, coronary arterial endothelium did not express HLA-DR antigens in the myocardial tissues of controls (n = 4). HLA-DR+ endothelial cells may play an important role in the development of Kawasaki vasculitis.