Pathological studies have revealed that the vasculitis in Kawasaki disease (KD) begins in the intima and the adventitia of blood vessels resulting in panvasculitis. In the present study we investigated the possibility that auto-antibodies to types III and II collagen are involved in the vascular injury. Seven out of 38 patients (18%) with KD had elevated serum levels of IgG antibody to type III collagen. None had increased serum antibodies to type II collagen. Specificity of the antibody to type III collagen in patients with KD was 86%. Three out of seven KD patients (42.9%) with coronary complications had elevated serum levels of the antibody. It is conceivable that the more severe the inflammation of systemic blood vessels, the more frequently antibodies to type III collagen are detected and coronary complications develop. Accordingly, this antibody may be secondarily detected in the serum of KD when systemic blood vessels are severely damaged. Detection of antibodies to type III collagen might be a risk factor predicting coronary complications.