INTRODUCTION: The association between idiopathic pulmonary fibrosis and patent foramen ovale has rarely been described. OBSERVATION: We report the cases of two patients, 72 and 59 years old, who presented with refractory hypoxemia in the context of pulmonary fibrosis. The hypoxemia was due to a right-to-left shunt through a patent foramen ovale (PFO), diagnosed by transoesophageal contrast echocardiography. The closure of the PFO allowed a decrease in the oxygen requirement in the first case: from 8 l/min to 3 l/min (PaO2 80 mmHg), and in the second case oxygen therapy could be stopped (PaO2 76 mmHg on room air). Right-to left shunts by PFO are usually associated with pulmonary arterial hypertension (systolic pulmonary arterial pressure at 70 mmHg for case 1), but in some cases the pulmonary artery pressure is normal (case 2), the shunt being due to an anatomical conformation. CONCLUSION: These two cases underline the importance of diagnosing right-to-left shunts in patients who have pulmonary fibrosis with severe hypoxemia, in order to reduce their oxygen needs.