Pulmonary hypertension is a condition in which the pressure in the pulmonary artery and lungs is too high. This puts stress on the right side of the heart because the muscles on the right side are not used to pushing blood out to the lungs against such high pressures. Over time, the right side of the heart is strained and begins to fail. This is a rare but serious condition that can develop in children or adults at any age. This fact sheet talks about Pulmonary Hypertension in children. For more information about Pulmonary Hypertension in adults, see the ATS Patient Information Series at www.thoracic.org/patients.