Liver disease affects the lungs. The majority of patients exhibit mild to moderate arterial hypoxaemia essentially attributable to an alteration in ventilation/perfusion matching and limited by an increase in ventilation. A minority (some 10%) of patients exhibit a "hepatopulmonary syndrome" defined by severe hypoxaemia with arterial PO2 below 60 mm Hg, dyspnoea, cyanosis, digital clubbing, orthodeoxia, platypnoea and demonstrable pulmonary vascular dilatations causing a true pulmonary shunt and a diffusion/perfusion imbalance. The hepatopulmonary syndrome is incurable but resolves over time after liver transplantation. An even lower proportion of patients, approximately 1%, develop pulmonary hypertension. Clinically this "portopulmonary hypertension" resembles primary pulmonary hypertension, with dyspnoea and fatigue as the main symptoms, histopathology and response to prostacyclin therapy. Portopulmonary hypertension is irreversible. Liver transplantation mortality in patients with portopulmonary hypertension ranges from 50 to 100%. The common cause of the hepatopulmonary syndrome and portopulmonary hypertension is portal hypertension and portosystemic shunting, indicating that vasoactive and angiogenetic factors originating from the liver normally control the pulmonary circulation.