Kawasaki disease occurs most frequently in Japan, where the disease was first discovered. In the United States, Kawasaki disease is the leading cause of acquired heart disease in children. It affects more than 4,000 children each year. Eighty percent of patients are younger than age 5. Kawasaki disease is a poorly understood illness. The cause has not been determined. It may be an autoimmune disorder. The disorder affects the mucus membranes, lymph nodes, walls of the blood vessels, and the heart. Kawasaki disease can cause inflammation of blood vessels in the coronary arteries, which can lead to aneurysms. An aneurysm can lead to a heart attack, even in young children, although this is rare.