The accuracy of a simple isotope method of assessing middle cerebral arterial occlusion was compared with that of transcranial Doppler ultrasound and with a careful clinical examination and CT brain scan to predict the probable pattern of cerebral arterial occlusion in 85 acute stroke patients. The isotope method detected likely middle cerebral artery main stem or large branch occlusion with equal accuracy to transcranial Doppler ultrasound. This simple isotope method is a practical way of assessing cerebral arterial occlusion, is inexpensive, non-operator-dependent, faster than transcranial Doppler ultrasound and uses technology which is already widely available. It may be useful for research in acute stroke at present and in future may help guide treatment of acute ischaemic stroke.