We report the case of a man who underwent a left-lung transplantation for pulmonary fibrosis and who developed deterioration of his respiratory condition at Day 16 post-operatively, with marked hypoxemia not explained by the usual early respiratory complications of lung transplantation. Contrast-enhanced transthoracic echocardiography identified a patent foramen ovale with massive spontaneous right-to-left shunting. As for the pathogenesis of this right-to-left shunting, we found no evidence supporting an elevation of right-side pressures, with the redirection of the inferior vena cava flow toward the patent foramen ovale being the suspected mechanism. We conclude that delayed reopening of the patent foramen ovale leading to massive right-to-left shunting is a possible complication after lung transplantation.