The Eisenmenger syndrome corresponds to the most advanced form of pulmonary arterial hypertension associated with congenital heart disease. The syndrome causes chronic hypoxemia, with an increase in erythrocyte mass, which predisposes to thrombotic complications. Pentoxifylline is a xanthine derivative and it is considered as a hemorrheological agent with described effects of reduction in erythrocyte and platelet aggregation, adhesion and activation of leukocytes, and endothelial damage. The main objective of this study is to verify if the chronic oral administration of pentoxifylline to Eisenmenger patients induces an increase in the circulating levels of thrombomodulin, a naturally occurring proteoglycan with anticoagulant, anti thrombotic and anti-inflammatory properties.