Abstract Body (Do not enter title and authors here): Introduction: The number of adults with congenital heart diseases (CHD) is rapidly increasing, who are frequently associated with cardiovascular morbidities and may result in poor outcomes. However, limited information is available regarding the current demographic status and the prognostic factors of adults hospitalized for the treatment of such patients on a nationwide scale. Hypothesis: We hypothesize that the prevalence of such CHD patients may increase with age in adulthood and that indices of loss to follow-up, emergency or non-referral admission, are related to poor outcomes. Methods: Using a nationwide claim-based dataset from Japan, we analyzed adult patients (aged 15 and over) with CHD diagnosed according to the ICD10 codes, who were hospitalized for the treatment of cardiovascular comorbidities between April 2013 and March 2022. Treatment groups included the surgery group and the catheter intervention (PCI) group for CHD or coronary revascularization and an ‘others’ group (patients treated for heart failure, infectious endocarditis or brain abscess), excluding those in the other two groups. CHD diagnosis was made based on the ICD-10 codes, while treatment categorization was made using specific procedure codes. Results: The study comprised 27,754 patients (male 49%), with a median age of 59 years (36-74) displaying bimodal peaks at <20 yo and 65-84 yo, and a body mass index of 21.6 (19.2-24.4). They were divided into 3 groups: surgery (n=8,800) (CHD surgery, n=8343; coronary bypass, n=457), PCI (n=3060) (CHD PCI, n=1998; coronary PCI, n=1062) and others (n=15,894) (heart failure, 96.9%). Underlying CHD by severity was categorized into simple (60.72%), intermediate (23.23%) and complex(9.44%) types. The overall patient cohort had emergency admissions (35.2%) and non-referral admissions (9.9%). Outcome measures included total hospital deaths (5.0%). In multivariate analysis, higher age, non-adult CHD center admission, CHD severity (intermediate or complex), emergency and non-referral admissions in the overall and the ‘others’ group correlated with hospital death (p<.001). The former 3 indices were independently associated with the latter 2 indices (p<.001). Conclusions: These findings demonstrated that higher age, underlying CHD severity, hospitals for admission, and indices of loss of follow-up were associated with poor outcomes. These findings underscore the importance of healthcare transition and CHD awareness in optimizing the outcomes.