Children with significant ASDs are characteristically slender of build and have a heart murmur. The murmur is caused by the extra blood flow across the pulmonary valve. Some children may experience shortness of breath or heart palpitations. However, they are normally active and show no other outward symptoms. There are no exercise restrictions for these children.

The larger the defect, the more likely children will have symptoms. Infants with a large ASD may develop congestive heart failure. However, if the defect is small (less than 2 millimeters), there is a very high probability that it will close on its own. Surgery is not usually performed in these cases.

Larger ASDs, which are more likely to remain open, cause an excessive flow of blood into the right atrium, right ventricle and pulmonary artery (see animation). This enlarges the right atrium and right ventricle (dilatation) and over years can cause high pressures in the pulmonary artery that will eventually distort its shape and may rarely damage the blood vessels in the lungs.

The enlargement of the right atrium can result in abnormal heart rhythms. These effects are not reversed by closing the ASD after the damage has been done. Heart failure is likely when a person with an untreated ASD reaches young adulthood.