Over a six months' period we prospectively studied the prevalence of patent foramen ovale (PFO) in patients under 50 years of age admitted for cerebral infarction as assessed by cerebral computed tomography. The study recruited 16 patients (9 male, 7 female, mean age 39 +/- 10 years). Before the contrast study, 5 patients had other identifiable sources of stroke after neurologic investigations and 2-D echocardiography with color-Doppler, whereas 11 had no precise origin. The overall prevalence was 50% with 8 PFO demonstrated, 6 during normal breathing and two during a coughing test. All were in the sub-group of 11 patients without a predetermined source of stroke (prevalence 73%) and in 83% of patients aged under 40. In accordance with the literature, we found a high prevalence of PFO in the ischemic cerebral infarction population aged under 50 years and without another possible source of stroke; this prevalence is higher in patients under 40 years old. This observation confirms the suspicion of paradoxical embolism in these cases.