In about 40% of young adult patients with ischemic stroke, no cause can be detected. Such ischemic strokes are called cryptogenic. Studies have shown a higher frequency of open oval window (foramen) in young patients with ischemic stroke than in healthy subjects. In spite of a statistically clear association, however, the pathophysiological mechanism for the development of cryptogenic ischemic stroke in the case of open oval window is not yet known. Transesophageal echocardiography is considered the standard method for establishing patency of the oval window, but due to the invasiveness and long duration of this procedure, its accessibility is relatively low. Right-to-left shunt can also be proved with transcranial Doppler sonography, which along with the use of a contrast agent and the Valsalva maneuver is a sensitive screening method for patients with open oval window. The sensitivity of this method is between 91 and 100%, while its specificity is 94%. In patients with cryptogenic ischemic stroke and positive results of transcranial Doppler sonography, antiaggregation prophylaxis is indicated, along with further diagnostic management, including transesophageal echocardiography. If the results are negative, the right-to-left shunt and therefore also open oval window can be excluded with a high probability. Due to its high specificity and sensitivity, this method can serve as a good screening test for detecting right-to-left cardiac shunts.