A patient with a large hydatid cyst of the liver developed a positionally symptomatic right to left shunting across a patent foramen ovale with both platypnoea and orthodeoxia, despite normal pulmonary arterial pressures and normal pulmonary function tests. When the patient was in the supine position the calculated right to left shunt was 15.1% and 29.5% when seated. The shunt was attributed to the compression of the right atrium and ventricle by the cyst. Surgical evacuation of the cyst relieved the symptoms and the positionally induced shunting.