We used the compressed spectral arrays (CSAs) of the electroencephalogram (EEG) to monitor cerebral blood flow related events in balloon test occlusion of the internal carotid artery (balloon Matas test). Reliability of the CSAs was examined in 22 patients subjected to the test. Of 9 patients who underwent subsequent permanent carotid occlusion, in 6 there was no change on CSAs and/or in neurological conditions. In 2 patients, there was a slowing on CSAs prior to the appearance of neurological deterioration during clinical testing. CSAs transformed these EEG changes into a succinct graphic display. In 1 of the 2, for whom the Matas test was repeated 1 year later, there was a change from positive to negative and delayed cerebral infarction occurred after carotid occlusion. In the other patient, an ischemic insult occurred during the balloon occlusive procedures. The third patient had a fatal delayed ischaemic complication 3 days after surgical ligation of the cervical carotid artery, despite the negative balloon Matas test. Complications in these 3 patients were presumably related to thromboembolic mechanisms. The balloon Matas test monitored by CSAs is useful for detecting change in brain functions attributed to an inadequate blood flow. However, this test is unreliable for predicting thrombo-embolic complications.