Patent foramen ovale (PFO), literally, open ovale window, is a persistent opening between the left and right atria of the heart. In fetal circulation, a major opening between the atria allows blood to largely bypass the lungs that are not yet being used for gas exchange. A flap normally closes over the opening after birth and is sealed by tissue. In approximately 25 percent of the population, a partial opening remains, the PFO. The opening can range in size from functionally irrelevant to physiologically significant, the latter allowing a substantial portion of blood to be shunting from the right heart to the left heart, bypassing gas exchange and filtration in the lungs. PFOs typically produce no symptoms and individuals are unaware of their status unless they are incidentally discovered through medical tests. However, the presence of a large PFO may increase the risk of DCS in divers who develop significant bubble loads. The correlation between PFO and DCS risk is not a clear one, since the frequency of PFO in the population is fairly high while DCS is relatively rare. The safest strategy — even if you have not been diagnosed with a PFO, but most certainly if you have — is to dive in a manner calculated to keep your bubble load low; this effectively eliminates any concern that bubbles might pass through a PFO and bypass the lungs, where they would normally be filtered out. The most commonly held consensus is that screening all divers for PFO is probably not warranted. And even in divers who have been diagnosed with a PFO, deciding whether it warrants surgical closure is a choice that each individual should consider carefully with a well-informed medical team.