Histopathological investigation of the vascular changes in Kawasaki disease was carried out on thirty-seven autopsied Japanese patients. Arterial lesions could be classified into following five stages from the view point of morphogenesis of arteritis; 1) endothelial degeneration and increased vascular permeability, 2) edema and degeneration of the media, 3) necrotizing panarteritis, 4) granulation formation, and 5) scarformation. Aneurysm with thrombus was observed mainly in the coronary artery in most patients. It is considered that the initial changes begins in the endothelial cells with increased vascular permeability. Platelet aggregation in the damaged endothelial cells seems to play an important role in the futher development of the arteritic changes. Vascular lesions were observed not only in the arterial system but also in the venous system, therefore Kawasaki disease is a systemic vasculitis rather than a systemic arteritis.