We compared the prevalence of patent foramen ovale (PFO), detected by two-dimensional contrast echocardiography, in a group of 60 adults aged under 55 who had experienced a cerebral ischaemic accident and had normal standard examination of the heart, and in a control group of 100 patients. The prevalence of PFO was significantly higher in neurological patients (40 p. 100) than in controls (10 p. 100; p less than 0.001). Within the neurological group, the prevalence of PFO determined blindly, i.e. without any knowledge of the aetiological diagnosis, increased with the uncertainty of diagnosis: 21 p. 100 when a cause could be determined (n = 19), 40 p. 100 when a facilitating factor of cerebral accident, such as mitral valve prolapse, migraine or consumption of oral contraceptives, could be identified (n = 15), and 54 p. 100 when neither cause nor facilitating factor could be found (n = 26; p less than 0.10). In view of the very high prevalence of clinically silent venous thrombosis, these results suggest that paradoxical embolism through a PFO might be responsible for cerebral ischaemic accidents more frequently than is generally believed.