The commonest heart defect ("Hole in the heart"). When small, such defects cause little in the way of problems and often "heal"  (close) on their own. Blood flows from left ventricle to right ventricle at high pressure, often producing a loud "murmur" with each heart beat. In most affected children no symptoms are present and the child grows normally and is fully active. Poor growth and / or breathlessness with feeding (in an infant), or reduced activity levels, in older children, may indicate that the defect is large enough to be putting some strain on the heart.

The effect on heart function depends on the size of the defect, but may be very minor. Surgery may be necessary for larger defects but may not be required, if the heart shows no evidence of "strain".

A "VSD" is present in more than 30% of children with congenital heart defects. Most defects are small and produce no symptoms. Complications occasionally develop and necessitate surgical repair.

The position of the VSD in the partition between the ventricles varies from one patient to another. Sometimes there are multiple defects.