The association between a patent foramen ovale (PFO) and thromboembolic events in young patients has been reported. Autopsy data suggest that a PFO may be present in 20 to 35% of the population. To further assess the role of a PFO in patients with possible thromboembolic events, precordial and transesophageal contrast echocardiography was performed in 104 consecutive patients (age range 16 to 84 years) presenting with a stroke, transient ischemic attack or peripheral artery embolus (group I). These patients were compared with 94 consecutive patients (age range 23 to 82 years) undergoing transesophageal echocardiography for other reasons (group II). A PFO was found in 22 patients; 9 of 35 (26%) with an event but no risk factor (group Ia), 10 of 69 (14%) with an event but a recognized risk factor (group Ib), and 3 of 94 control patients (3.2%) (group II) (group Ia vs II: relative odds 10:1, p less than 0.001; group Ib vs II: relative odds 5:1, p less than 0.01; and group Ia vs Ib: p = not significant). The detection of a PFO was not related to age. The relatively low prevalence of a PFO in this study may reflect patient selection, but other explanations include: (1) Transesophageal contrast echocardiography may be relatively insensitive for its detection; (2) the prevalence in the general population may have been overestimated; and (3) most PFOs are very small, clinically insignificant and undetectable with this technique.(ABSTRACT TRUNCATED AT 250 WORDS)