 Purpose: High blood pressure in the lungs or pulmonary hypertension PH is a common complication of fibrosing or interstitial, ILD lung disease. When present, it is associated with markedly reduced prognosis and survival. Endothelin-1 ET-1is over-expressed in patients with PH and ILD, and is thought to play a role in the development of both conditions. Bosentan blocks the action of ET-1, and has been shown to be beneficial in patients with PH from an unknown cause, or related to other conditions such as heart conditions, connective-tissue disease, and HIV. It is important to establish whether bosentan treatment also benefits patients with PH and ILD. 