It has now become possible to close a patent foramen ovale (PFO) using a percutaneous device. In addition, it has become increasingly clear that right-to-left shunting through a PFO can cause both stroke and decompression illness, due to paradoxical embolism of blood clots or gas bubbles. For these reasons, diagnosis of large PFO with significant right-to-left shunts has become important. The diagnosis can be made by transthoracic echocardiography with injection of bubble contrast, combined with multiple sustained Valsalva manoeuvres. Whilst transoesophageal echocardiography provides detailed anatomical information, functional information (with regard to right-to-left shunting) is better provided by transthoracic studies where a Valsalva can be properly performed. Device closure can prevent right-to-left shunting and can be achieved using a number of different devices. However, device closure has yet to be proven beneficial in a randomised trial. In light of the clear evidence implicating PFO, we undertake closure procedures in selected patients.