Incomplete Kawasaki disease (IKD) and atypical Kawasaki disease (AKD) represent rare conditions. Two cases of unexpected or sudden infant death are reported. The diagnosis for a 3-month-old girl was determined by echocardiography, and the child unexpectedly died despite appropriate treatment, whereas autopsy determined the diagnosis of AKD for a 4-month-old boy. In both patients, giant coronary artery aneurysms with thrombosis and vasculitis, myocarditis, and coagulative necrosis were shown at autopsy. These rare forms of IKD and AKD carry a poor prognosis and represent a paradox between the severe cardiovascular damage and the clinical presentation that mimics common and usually self-limiting exanthematic infectious disease in infancy.