The diagnosis of brain death is important for many reasons. Clinical testing may not be feasible, so that cerebral angiography is needed to confirm circulatory arrest. Angiography is, however, cumbersome and expensive. We present the results of transcranial Doppler (TCD) studies on the middle cerebral arteries of 40 patients with brain death. In six, no cranial signals could be obtained. All except two of the remaining patients had typical TCD appearances, with a reverberating pattern and little or no net forward flow. One patient had this appearance on one side and a carotid-cavernous fistula on the other side, and another had preserved middle cerebral artery flow. Sixteen control patients all had quite different signals, with some showing evidence of raised intracranial pressure and some of vasospasm. There were thus few false-negative results, and more importantly no false-positives. Refinement of this technique, especially looking to a numerical value for the net flow velocity below which circulatory arrest is certain, is needed. The possibility of a changing pattern on serial studies, with prediction of brain death before it actually occurs, is also to be explored.