An extra-intracranial anastomosis operation was carried out in 60 stroke patients in order to bypass the occlusion or severe stenosis in the internal carotid or middle cerebral artery. Postoperative angiographical follow-up studies showed a well functioning extra-intracranial bypass in more than 90 % of the patients. In many cases repeat angiography done weeks or months following surgery revealed an enlargement of the caliber of the extracranial artery used for constructing the bypass and also of the cerebral arteries that were irrigated by the bypass. Furthermore we noted that the new collateral channel was able to fill a steadely increasing part of the cerebral circulation and that it was also found to irrigate territories of the brain that were previously well perfused by leptomeningeal anastomosis or retrograd flow through the ophthalmic artery etc. resulting in a reversal of the direction of blood flow in these collaterals. The neurological follow-up examinations revealed that the increase in cerebral blood supply following the extra-intracranial anastomosis operation resulted in a considerable improvement of the post-stroke neurological deficits.