The authors present three cases of AION with suspected temporal arteritis to show why a Doppler sonographic examination should be performed prior to a biopsy of the temporal artery. In Case 1, with stenosis of the internal carotid artery, the cerebral perfusion depended, entirely on collateral circulation, the nutritive vessel of which was the temporal artery. Cases 2 and 3 illustrate a proximal stenosis of the ophthalmic artery. In these two cases the temporal artery maintained the blood supply to the eye. All these findings were obtained by Doppler sonographic examinations. Biopsy of the temporal artery, which could otherwise have caused sudden death in Case 1, or blindness in Cases 2 or 3, was therefore dispensed with.