OBJECTIVES: To find a correlation between the patent foramen ovale (PFO) size measured by the sizing balloon and the appropriate closure device size. METHODS: The PFO of 57 patients was closed using a sizing balloon. A mathematical model was introduced to relate the PFO balloon waist diameter to the closure device size based on the PFO transformation from a slit-like to a circular form during balloon inflation. According to this model, PFOs smaller than 8 mm should be closed with a 25 mm device, PFOs 8 mm to 11 mm with a 35 mm device, and PFOs larger than 11 mm with an Amplatzer septal occluder. In the first group, 36 patients (63.2%) received an appropriately sized device and six patients (10.5%) received an oversized device. In the second group, 15 patients (26.3%) received an undersized device. RESULTS: A comparison of the PFO dimensions in two views showed that the PFO slit was circular when the balloon was inflated. A six-month echocardiography follow-up was obtained in 46 patients (80.7%). Five patients (13.9%) in the group with an appropriately sized device had a discrete residual shunt during Valsalva. In the second group, five patients (33.3%) had a residual shunt (P = 0.06), of which one was considered large. CONCLUSION: The sizing balloon is helpful in selecting the PFO closure device size. Consequently, the incidence of residual shunt and recurrent events may be reduced.