Background: Sporadic cases of acute coronary syndrome (ACS) have been reported in adults with coronary sequelae presumably due to Kawasaki disease (KD). However, ACS in adults with a confirmed (whether followed-up or lost to follow-up) as well as unconfirmed history of KD is poorly characterized. Methods: To investigate ACS in such adults after KD during 2000-10, a nationwide survey was conducted in Japan. Results: A total of 67 patients (median age 35 yo, male 76%) were recruited. A diagnosis of KD was made in 32 during acute illness (Group A), in which 17 were lost to follow up. A KD diagnosis was made retrospectively in the other 35 patients from coronary imaging at ACS (Group B). Overall, 67 patients were characterized by demonstrable thrombosis (74%) as well as the presence of giant aneurysms (GAN) (≥8mm) (38%), severe stenosis (>75%) (38%) and IVUS-derived calcification (92%) in culprit lesions. Group A represented younger age at ACS (26.5 yo vs. 40 yo in group B, p<.001), lower conventional coronary risks (87% vs 65%, p=.043), low percentage of severe stenosis (25% vs. 50%, p=.302) and high proportion of IVUS-derived calcification (86% vs. 100%, p=.335). In group A, patients who were followed up before ACS represented a higher proportion of GAN in culprit lesions (69% vs. 29% in KD patients lost to follow up, p=.030) as well as a shorter interval from acute KD (20 y vs. 27 y, p=.008) and higher percentage of medication before ACS (87% vs. 0%, p<.001). In the convalescence of acute KD in group A, the vessel size of prospective culprit lesion was 6.0-7.9mm in 36% and ≥8mm in 64%. Coronary angio, IVUS and MDCT of cases in 3 categories are presented. Conclusions: Premature ACS in young adults with a confirmed history of KD, significant coronary sequelae just after acute KD and low coronary risks is emerging in Japan. ACS in this population comprises two subtypes: ACS with GAN in culprit lesions in adults followed up for KD, as in childhood myocardial infarction in KD, and ACS in adults lost to follow up, characterized by the absence of GAN or severe stenosis and the presence of IVUS-derived calcification in culprit lesions, which was unmasked in this population. The present findings may give an insight into mechanisms in ACS, screening and medication, as well as recognition of KD in adulthood.