Idiopathic pulmonary arterial hypertension (PAH) is included in Group 1 of PAH classification, while pulmonary veno-occlusive disease (PVOD), pulmonary capillary hemangiomatosis (PCH), portopulmonary hypertension (PoPH) and HIV-associated PAH constitute the subgroups of Group I PAH disorders. PVOD has similar clinical presentation, hemodynamic characteristics and genetic background with PAH although main pathology is in venules and definite diagnosis is done by biopsy although not recommended due to high risk of bleeding. PCH, a rare idiopathic lung disorder, causes PAH in young adults and mostly is diagnosed after death by autopsy. PoPH is defined as PAH due to primary chronic liver disease associated with intrapulmonary vascular dilatations and hypoxia. HIV associated PAH is a rare complication of HIV-infection, and it is seen especially in HIV-infected patients with intravenous drug abuse.