BACKGROUND: A patent foramen ovale (PFO) has a complex anatomy, and evaluating the size before closure may be challenging. We aimed to investigate the correlation between preprocedural transesophageal echocardiography (TEE) and balloon sizing of PFO in patients undergoing percutaneous PFO closure. METHODS: A retrospective single-center study with analysis of 100 patients who, due to paradox thromboembolism in the left circulation, underwent percutaneous PFO closure. The PFO sizing was compared to measures attained by TEE and balloon sizing using linear regression analysis. RESULTS: PFO size measured by TEE occurred smaller than balloon sizing (2.19 mm [95% CI: 1.91 to 2.46] vs. 8.51 mm [95% CI: 8.02 to 9.00], CONCLUSIONS: PFO defect and shunt size measured by TEE showed a poor correlation with balloon sizing. Neither PFO channel length nor septal mobility were correlated to the PFO size measured by balloon sizing.