Atrioventricular septal defect accounts for approximately 3% of all congenital cardiovascular malformations. We describe a case of a 34-year-old woman with inoperable sinus venosus atrial septal defect with severe pulmonary hypertension and symptoms of the Eisenmenger syndrome. Electrocardiography, chest radiography, transthoracic and transoesophageal echocardiography with and without contrast were performed and provided valuable information about haemodynamic consequences of this malformation.