Patients who have suffered a transient ischaemic attack or minor stroke are examined thoroughly to determine the aetiology of the cerebrovascular incident. An examination of the carotid arteries is compulsory to detect significant stenosis. In some patients, cardiogenic sources of emboli may be suspected. This study examined the possibilities of screening for patent foramen ovale (PFO) using i.v. injection of a microbubble solution with Doppler monitoring of the internal carotid artery (ICA) and the middle cerebral artery (MCA). Transoesophageal echocardiography (TEE) served as the gold standard. Twenty-three patients with a recent episode of cerebral ischaemia were examined: 12 patients with and 11 without PFO documented by TEE. After i.v. injection, microbubbles were monitored visually in the right and left atria and using Doppler sonography in the right ICA and in the right MCA. In the group with a PFO (n = 12), microbubbles were detected in the ICA in seven patients (7 of 12) and in the MCA in 10 patients (10 of 10). In the group without a PFO, microbubbles were detected in the ICA in one patient and in the MCA in two. This corresponds to a sensitivity and specificity, respectively, of 58% and 91% for the ICA/duplex technique and 100% and 82% for the MCA/transcranial Doppler technique. From this limited sample, it is shown that transcranial Doppler monitoring of the middle cerebral artery, but not ultrasound duplex monitoring of the internal carotid artery, can be used as screening for patent foramen ovale.