Paradoxical embolism by way of left-to-right shunt (LRSh) may be underestimated as a cause of stroke in young adults. We studied the prevalence of LRSh in 58 patients under 45 years of age. The sensitivity of transcranial Doppler ultrasound (TDU) with contrast medium and transthoracic echocardiography (TTE) with contrast for diagnosing LRSh, and the clinical and radiological signs of stroke are analyzed. TDU with contrast medium allowed shunt to be identified in 34.5%, whereas TTE identified 19%. The prevalence of LRSh was significantly higher in patients with cryptogenic stroke (p = 0.0043) and in patients without vascular risk factors (p = 0.0069). The group with shunt manifested less severe neurologic impairment both upon admission to the hospital and upon release. TDU with contrast medium is an excellent tool for diagnosing LRSh and a useful technique for studying cerebral infarction of uncertain origin.