Additional Study Information: This study is being done to determine whether early combination therapy with two study drugs called sildenafil (REVATIO) and bosentan (TRACLEER) can improve pulmonary hypertension when compared to using sildenafil alone. Pediatric pulmonary arterial hypertension (PAH) is a severe medical problem that often worsens with time. It is caused by problems with the growth and/or function of the arteries that supply blood to the lungs. This is a growing problem in the world today. Despite the serious nature of this disorder, the medical field does not yet know the best first treatment for infants, children, and adolescents with PAH. Sildenafil and bosentan are medications that work in two different ways that treat pulmonary hypertension by relaxing the muscle cells that are in arteries in the lung. These medicines allow more blood flow to the lungs. Lead Principal Investigator: Lewis Romer, MD John Hopkins University IRB00300590.