Several reports have described populations of divers with decompression sickness (DCS) who have a patent foramen ovale (PFO). The presence of a PFO is known to occur in about 30% of the normal population, hence 30% of divers are likely to have a PFO. Although observations have been made on the presence of a PFO in divers with and without DCS, the risk of developing DCS when a diver has a PFO has not been determined. In this study, Logistic Regression and Bayes' theorem were used to calculate the risk of DCS from data of three studies that reported on echocardiographic analysis of PFO in a diving population, some of whom developed DCS. Overall incidence of DCS was obtained from the sport diving population, from the U.S. Navy diving population, and from a commercial population. The analysis indicates that the presence of a PFO produces a 2.5 time increase in the odds ratio for developing serious (type II) DCS in all three types of divers. Since the incidence of type II DCS in these three populations averages 2.28/10,000 dives, the risk of developing DCS in the presence of a PFO remains small, and does not warrant routine screening by echocardiography of sport, military, or commercial divers.