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 25 children with KD and enlarged coronary arteries for two 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.