Abstract Background Previous studies in adult congenital heart disease “CHD” have demonstrated a link between renal dysfunction and mortality. However, the prognostic significance of renal dysfunction in CHD and decompensated heart failure “HF” remains unclear. We sought to assess the association of renal dysfunction and outcomes in adults with CHD presenting to our center with acute HF between 2010-2021. Design & Methods This retrospective analysis focused on the association between renal dysfunction, pre-existing and on admission, and outcomes during and after the index hospitalization. Chronic kidney disease “CKD” was defined as eGFR<60 mL/min/1.73m2. Cox regression analysis was used to identify predictors of death post-discharge. Results In total, 176 HF admissions were included (mean age 47.7±14.5 years, 43.2% female). One-half of patients had CHD of great complexity, 22.2% had a systemic right ventricle, and 18.8% Eisenmenger syndrome. CKD was present in one-quarter of patients. The median length of intravenous diuretic therapy was 7[4-12] days, with a maximum dose of 120[80-160] mg furosemide equivalents/day, and 15.3% required inotropic support. In-hospital mortality was 4.5%. The 1- and 5-year survival free of transplant or ventricular assist device “VAD” post-discharge were 75.4% (95%CI: 69.2-82.3%) and 43.3% (95%CI: 36-52%), respectively. On multivariable Cox analysis, CKD was the strongest predictor of mortality or transplantation/VAD. Greatly complex CHD and inpatient requirement of inotropes also remained predictive of an adverse outcome. Conclusions Adult CHD patients admitted with acute HF are a high-risk cohort. CKD is common and triples the risk of death/transplantation/VAD. Expert multidisciplinary approach is essential for optimizing outcomes.