Severe cyanosis due to pulmonary arteriovenous fistulas occurs often after a bidirectional superior cavopulmonary anastomosis (Glenn operation) and also in some congenital anomalies in which hepatic venous blood bypasses the lungs in the first passage. Relocation of hepatic flow into the lungs usually causes these fistulas to disappear. Similar pulmonary arteriovenous fistulas are observed in hereditary haemorrhagic telangiectasia, and in liver disease (hepatopulmonary syndrome). There is no convincing identification yet of a responsible hepatic factor that produces these lesions. Candidates for such a factor are reviewed, and the possibility of angiotensin or bradykinin contributing to the fistulas is discussed.