Hepatopulmonary syndrome (HPS) is the clinical relationship between hepatic disease and the existence of pulmonary vascular dilatations, which can result in a range of arterial oxygenation abnormalities. It is probably caused by an alteration in the synthesis or metabolism of vasoactive pulmonary substances at a hepatic level, leading to vasodilatation of pulmonary vessels and diffusion perfusion defects. The Abernethy malformation is characterized by the congenital diversion of portal blood away from the liver, by either end-to-side or side-to-side shunt. Here, we report on a 5-year-and-11-month-old-boy who had started cyanosis at age 4 years and 11 months, and did not have any other pulmonary or cardiac signs or symptoms. In the investigation, arterial blood gases revealed a PaO2 of 41.4 mm Hg. The chest x-ray film and echo Doppler cardiography were normal, Nuclear scanning with Technetium 99m-labeled macroaggregated albumin showed the presence of arteriovenous shunt, at 47%. Abdominal echography revealed Abernethy malformation with an absence of portal vein. We concluded that the patient had HPS caused by Abernethy malformation, The possible mechanism is that in this malformation, there is a deviation in the blood that comes from the spleen to the vena cava without passing through the liver, so there is no metabolism of some substances which can be responsible for the imbalance between the vasodilatation and the vasoconstriction of the pulmonary circulation. Abernethy malformation must be included as one of the etiologies of hepatopulmonary syndrome. This is the first case described in the literature with this form of presentation. (C) 2002 Wiley-Liss, Inc.345391394