Stenosis means narrowing. The pulmonary valve normally has three leaflets and may have only one or two leaflets, or three leaflets which may be malformed or completely stuck together. These abnormal leaflets do not open properly, causing a narrowing of the valve. The right ventricle works harder to pump blood past the deformed valve. The extra stress put on the right ventricle may cause 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 can cause symptoms in infancy such as rapid breathing, poor feeding, and no energy. In some cases, cyanosis (bluish color of lips, skin and nails) may develop.