A complete description of haemodynamics and gas exchange before and after percutaneous coil embolization of multiple pulmonary arteriovenous malformations is reported in a 45 year old woman with hereditary haemorrhagic telangiectasis (HHT). Before treatment, whilst the patient complained of severe dyspnoea during daily activities, an intrapulmonary shunt of 31% was measured (inert gas elimination technique), together with a cardiac output (thermodilution technique) of 12.4 L.min-1, resulting in a resting arterial oxygen tension (PaO2) of 8.53 kPa. Effective occlusion of all visible pulmonary malformations resulted in a rapid and major improvement in exercise tolerance, whilst resting PaO2 remained almost unchanged. A second investigation performed 4 months after treatment revealed a persistent intrapulmonary shunt of 19%, a cardiac output of 7.35 L.min-1, and a resting PaO2 of 10.53 kPa. We conclude that major increases in cardiac output largely contribute to the maintenance of PaO2 in patients with multiple pulmonary arteriovenous malformations and intrapulmonary shunt. The benefit of coil embolization is due both to an improvement in arterial oxygenation and a normalization of cardiac output.