To define the prevalence rate of left-to-right interatrial shunt through patent foramen ovale in adults with symptomatic left-sided cardiac lesions, 56 patients were examined with transthoracic and transesophageal echocardiography and cardiac catheterization. By transesophageal echocardiography, 15 patients (group A) were found to have left-to-right interatrial shunt through patent foramen ovale, constituting a prevalence rate of 27%. In group A, transthoracic echocardiography detected the interatrial shunt in two patients, and catheterization detected it in only one. The remaining 41 patients (group B) had no shunt demonstrated by either echocardiographic or catheterization examinations. In another 44 patients (group C) with no significant left-sided cardiac lesions, no interatrial shunt could be found by transesophageal echocardiography even though some of them had a patent foramen ovale. All patients with such shunt had left atrial size > 34 mm (45.1 +/- 6.0 mm), left atrial pressure > 13 mm Hg (23.4 +/- 7.6 mm Hg), and pressure gradient between left and right atria > 10 mm Hg (19.3 +/- 5.2 mm Hg). These findings support the concept that in the presence of a patent foramen ovale any left-sided cardiac lesion increasing left atrial size and pressure may induce left-to-right interatrial shunt through this channel and that the prevalence rate is much higher than generally acknowledged.