An increasing number of patients with congenital heart disease (CHD) are entering adulthood. Although prior studies have focused on the causes of death in the pediatric population, the modes of death for adults with CHD have not been well defined. In a cross-sectional study performed on a population of 2,609 consecutive adults assessed at a CHD specialty clinic, there were adequate information available in 197 of 199 deceased patients. Mean age at death was 37 ± 15 years. Mortality was highest in patients with congenitally corrected transposition of the great arteries (26%), tricuspid atresia (25%), and univentricular connection (23%). Youngest mean age at death was observed in patients with tricuspid atresia (27 ± 5 years), complete transposition of the great arteries (27 ± 7 years), pulmonary atresia (27 ± 6 years), and aortic coarctation (29 ± 6 years). Sudden death (26%) was the most common cause of death followed by progressive heart failure (21%) and perioperative death (18%). Postmortem examinations were performed in 77 of 197 deceased patients (39%) and provided incremental data on the mechanism of death in 22% of autopsies. Thus, the 3 major causes of death in the growing population of adults with CHD are sudden, perioperative, and progressive heart failure.