To determine the accuracy of the posterior cerebral radionuclide angiogram (CRAG) in the detection of carotid occlusive disease, a retrospective study of 186 patients who underwent both CRAG and contrast cerebral angiography within a period of 30 days was carried out. Asymmetry of neck activity was highly specific (97%) but insensitive (27%) for detection of carotid disease. When asymmetry of cerebral perfusion was also considered, the sensitivity rose to 61%, but the specificity fell to 77%. For the entire group, including carotid and intracerebral disease, the sensitivity of the posterior CRAG was 49% and the specificity 89%. From these data and a review of the literature, we conclude that the CRAG is too insensitive to be used as a screening test for carotid occlusive disease.