Atrial septal defect is the most frequently encountered major congenital cardiac disorder in the adult population, with a prevalence of 0.2 to 0.7 per thousand. Several patients tolerate large unrepaired defects for 80 years or even longer without serious disability. However, it is assumed that, as a rule, atrial septal defect reduces life expectancy, the average age at death not exceeding 50 years. This estimation is based on studies derived mainly from necropsy series or from the admission profile of patients undergoing late operative repair. The onset of atrial fibrillation, with an incidence ranging from 13 to 52 percent among patients older than 40 years, as well as the progression of pulmonary arterial hypertension in up to 53 percent of patients, results in congestive heart failure and functional limitation. On the other hand, very few longitudinal studies thus far have directly and systematically followed the course of adults with unrepaired defects. Thus, many issues regarding the natural history and prognosis of atrial septal defect still remain unresolved. Follow-up series of older patients with nonoperated defects could yield valuable information even in an era when routine early surgical closure is increasingly being recommended.