30 patients with a history of decompression sickness were examined for the presence of patent foramen ovale by bubble contrast, two-dimensional echocardiography and colour flow doppler imaging. With bubble contrast, 11 (37%) of the patients had right-to-left shunting through a patent foramen ovale during spontaneous breathing. 61% of a subset of 18 patients with serious signs and symptoms had shunting. This number was significantly higher than the 5% prevalence seen with the same diagnostic technique in 176 healthy volunteers. The presence of patent foramen ovale seems to be a risk factor for the development of decompression sickness in divers.