This study will evaluate the safety and potential efficacy of inhaled CO in subjects with severe PAH. Over forty subjects with severe PAH despite best available therapy will be screened from the UIC pulmonary vascular disease clinic, of which twenty subjects will be recruited for participation in the trial. The trial will consist of an initial screening period to determine subjects' eligibility for the study. This will be based on a previous echocardiogram, a six minute walk test and right heart catheterization done as part of standard care for subjects with pulmonary arterial hypertension. Following the initial screening, the trial will last sixteen weeks, during which subjects will receive inhaled carbon monoxide up to three times weekly at the University of Illinois at Chicago.