Congenital heart disease (CHD) is frequently described in patients with Down syndrome (DS). Advances in cardiovascular surgery have led to an increase in life expectancy for adult patients with CHD. Patients with DS and CHD that have undergone early surgical repair are now living well into the 7th decade of life. This case report illustrates the survivability of an unrepaired 64-year-old woman with DS and CHD of moderate complexity. Transthoracic echocardiogram confirmed diagnosis of transitional atrioventricular septal defect (AVSD). Prior to the early 1990s, patients with DS did not undergo routine surgery for complex CHD. Patients with DS and unrepaired transitional AVSD are at high risk for developing late-onset complications including pulmonary vascular disease, left ventricular outflow tract obstruction, and valvular regurgitation. Appropriate lifelong evaluation and surveillance of late-onset complications in adults with CHD and DS will ultimately lead to the highest quality of care.