Abstract: AIM To explore the method to determine the size of patent foramen ovale (PFO). METHODS Forty-seven patients, at the age of 9 to 68 years (42±17 years), were included in this study. The PFO size was measured by transesophageal echocardiography (TEE) and the correlation between the volume of right-to-left shunt (RLS) by transthoracic echocardiography (cTTE) and the balloon stretched diameter (BSD) for PFO was investigated. RESULTS In the resting state, the PFO diameters measured by TEE were 3±1mm for patients without PFO-RLS and (5±1) mm for patients with PFO-RLS. cTTE measurement showed that in the resting state the PFO size of patients with RLS was larger than that of patients without RLS (P<0.01). The maximum sizes of PFO measured by TEE were (4±1) mm and the balloon stretched diameters were (9±4) mm. The BSD was significantly related to the maximum size of PFO measured by TEE (r=0.98, P<0.01) and was much larger than the measured maximum size of PFO measured by TEE (P<0.01). In patients with simple PFO, there was a tiny difference between the maximum size of PFO measured by TEE and the BSD, indicating a higher correlation between them. In patients with complex PFO, the value measured by TEE was significantly smaller than the BSD, where a lower correlation was indicated. CONCLUSION The more the volume of RLS is in the resting state, the larger the PFO size measured by TEE and the BSD are. The maximum size of PFO measured by TEE is significantly smaller than the BSD. The maximum size of PFO measured by TEE, together with cTTE measurements of RLS, is beneficial to the determination of large PFO.