Objective: Cost-of-illness studies in Adult Congenital Heart Disease (ACHD) have mainly been limited to hospitalizations. This is the first paper to provide a comprehensive overview from a societal perspective including inpatient and outpatient medical costs, and absenteeism-and unemployment-related societal costs. Methods: A retrospective longitudinal (2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015) database analysis was performed in Belgium combining administrative and clinical databases (n = 10,572). Trends in resource use and costs per patient year were standardized to assess the impact of changes in the patient population composition. Generalized Linear Mixed Models assessed the impact of age, sex, lesion complexity, and time. Costs were converted to 2018 values. Results: Medical costs per patient year increased from V3490 to V4536 with a milder increase in patients with severe lesions. Although unemployment-related costs decreased, total societal costs increased due to more long-term (≥1 yr) invalidity. An increase in long-term invalidity was particularly found in patients ≥30 yrs and in patients with mild or moderate lesions. Resource use (e.g., dental care, nursing care, physiotherapy, emergency department) increased substantially in all patient groups over time. The annual percentage of patients with severe lesions receiving any cardiac and specialized cardiac follow-up increased with respectively 11 and 13 percent points to 81% and 52%, with a simultaneous decrease in hospitalization rate. Conclusion: Medical cost increases in ACHD are most pronounced in patients with mild and moderate lesions, relatable to their higher age. Economic data are necessary to allocate