The pulmonary artery is the major blood vessel which carries "blue blood", low in oxygen, from the right side of the heart\ to the lungs. The pulmonary valve is located at the beginning of the pulmonary artery. It opens and closes to regulate blood flow into the pulmonary artery. The normal pulmonary valve has three parts (leaflets) is flexible, and allows blood to flow only in one direction. When the normal pulmonary valve is open, blood flows easily into the pulmonary artery; when closed, no blood leaks back into the right ventricle.

Stenosis means narrowing. Pulmonary stenosis can occur in several ways: the valve may have only two leaflets which are partially stuck together or three leaflets which may be thick and partly or completely stuck together. These abnormal leaflets do not open completely, causing narrowing of the valve. The right ventricle works harder to pump blood past the narrow valve. With time, the extra work may stress the right ventricle causing it to enlarge.

Pulmonary stenosis can vary from mild to severe. Children with mild pulmonary stenosis usually have no symptoms. If the stenosis is very mild, surgery is usually not needed. Severe stenosis, however, can cause symptoms in infancy such as rapid breathing, poor feeding, and poor energy. In rare instances, cyanosis (bluish color of lips, skin, and nailbeds) may develop. In these cases, balloon dilatation of the pulmonary valve is necessary. Some cases may even require surgery.

Children with pulmonary stenosis require penicillin (also called SBE prophylaxis) prior to any dental work or surgery on the mouth, bowel, or bladder. This helps prevent the uncommon, but possible, occurrence of bacterial infection affecting the heart near the abnormal valve.