 Over time, patients with fibrosing or interstitial lung disease ILD can develop high lung blood pressures pulmonary hypertension, and this is associated with poorer prognosis and survival. It is thought that development of PH contributes to the deterioration and death of patients with ILD. Endothelin-1 ET1 is a substance contributing to the development of both PH and ILD. Bosentan is a drug blocking the action of ET-1 by binding to its receptors. Bosentan clearly benefits patients with PH of unknown cause, or related to other diseases such as heart conditions, or HIV both alone and in combination with other treatments. In patients with fibrosing lung disease and PH, there have been no controlled treatment studies. Clearly it is important to evaluate the effectiveness of bosentan in these patients. 