Kawasaki disease (KD) affects infants and young children causing inflammation of the skin and blood vessels including the coronary arteries of the heart. Despite the currently available therapy, about one third of children develop enlargement of the coronary arteries that can lead to serious complications such as coronary artery stenosis, heart attack and even death.||Kawasaki disease is the most common heart disease in children in the USA and it is especially common among the children of Hawaii. Every year, 50-90 children are diagnosed with KD in Hawaii and unfortunately there is no medication available to successfully prevent coronary artery damage in a subset of cases.||During the first few weeks of the illness, cells of the immune system attack the coronary arteries and release a special substance (MMP) that is responsible for the coronary artery enlargement. There is a common antibiotic, doxycycline that can specifically block the action of this special substance (MMP). Research done on animals with KD showed that doxycycline was able to block this special substance and prevent enlargement of coronary arteries. Research in adults with enlargement of the main artery in their abdomen also showed that doxycycline may improve the outcome. Based on these studies doxycycline may be a promising therapy for children with KD, who develop enlargement of the coronary arteries.||The investigators' proposed research study will assess the usefulness of doxycycline in preventing the progressive enlargement of coronary arteries in children with KD. The investigators plan to perform a small (pilot) study to evaluate how good is doxycycline in preventing coronary artery enlargement. The investigators will treat 50 children with KD and enlarged coronary arteries for three weeks with doxycycline and assess the change in coronary arteries as well as the blood levels of the special substance (MMP). If doxycycline proves to be beneficial in this small study, the investigators are going to design a large research study involving multiple institutions on Hawaii and the mainland and will recruit more children to be certain about the value of the proposed treatment. The investigators' proposal may change the treatment protocol of KD and could present a possible treatment for children with enlarged coronary arteries preventing potentially devastating consequences.