Background: A patent foramen ovate (PFO) is not widely recognized as a factor contributing to hypoxemia in patients with chronic obstructive pulmonary disease (CCPD). We therefore sought to clarify the prevalence and clinical significance of a PFO in patients with COPD, and to analyze the factors related to its occurrence. Methods: This study included 52 consecutive stable patients with COPD and 50 healthy controls. The demographic and clinical features of the study group were noted. To test for a PFO, standard and contrast transthoracic echocardiographic examinations were performed while resting and during the Valsalva maneuver (VM). Patients performed 6-min walking tests (6 MWT), and the distances traveled were measured.