Pulmonary arterial hypertension (PAH) is a type of high blood pressure in the arteries that carry blood from the heart to the lungs. PAH occurs when the openings in the blood vessels of the lungs get smaller and smaller. These smaller openings can be caused by the following:

* The walls of the arteries tightening
* The walls of the arteries becoming stiff and narrow from an overgrowth of cells The increased pressure in the pulmonary arteries strains the right side of the heart and it begins to fail, causing difficulty breathing and other symptoms. As PAH progresses, symptoms get worse.

There is no cure for PAH, but several medications like endothelin receptor antagonists (ERAs), prostacyclin analogues (PCAs) and riociguat, a soluable guanylate cyclase stimulator, are available to help slow the progression of changes in the pulmonary arteries and help reduce symptoms. Riociguat can be taken together with ERAs and PCAs.

In this study, the researchers want to learn about how well riociguat works, how safe it is when patients take it in 1 of these ways:

* alone
* with ERA
* with PCA
* with ERA and PCA The dosage for each patient will be decided by their doctor. The researchers will review information collected from the patients who have decided with their doctor to start riociguat treatment for their PAH. The study will include about 500 patients in the United States who are at least 18 years old. All of the patients will have either just started taking riociguat or will have been taking it for less than 3 months No investigational products will be administered in this study. Patients will be treated with the Standard of Care (SOC) for PAH. The SOC is the currently appropriate treatment in accordance with scientific evidence and agreed upon in collaboration between medical experts for PAH. There will be no study-mandated visits or treatments.

The patients will be in the study for up to 2 years. During this time, they will visit their doctor every 3 to 6 months as part of the Standard of Care. At these visits, the patients will answer questions about their PAH symptoms and whether they have any medical problems. They will also do exercise tests to see how well they are able to breathe and how tired they get while exercising. The doctors will perform other usual examinations which are part of the Standard of Care such as echocardiograms (images of the heart to show how the heart is working) and a right heart catheters (to measure the pressures in the heart) and will take the usual blood and urine samples.