Background: \- High blood pressure in the lungs, known as pulmonary arterial hypertension (PAH), is a rare disorder. Some people have disease-associated PAH and some have PAH from an unknown cause. Researchers want to follow the natural history of all PAH patients to understand how PAH progresses in order to discover targets for future research into new treatments. To further identify treatment targets, they will compare healthy volunteers to patients with PAH.

Objectives: \- To study the natural history of PAH.

Eligibility: * Individuals at least 18 years of age who have PAH. * Healthy volunteers at least 18 years of age.

Design: * Participants with PAH will have periodic visits to the National Institutes of Health Clinical Center. After the first visit, they will return in 6 months and then yearly or every other year for as long as the study continues. * The first visit will take up to 3 days. It will involve the following tests: * Physical exam and medical history * Blood and urine samples * Heart and lung function tests and imaging studies * Six-minute walk test * Questions about exercise and physical activity * Healthy volunteers will have only one visit to the Clinical Center, during which they will undergo screening tests, and complete many of the same tests as patients with PAH