We obtained scintigraphic images of the neck from 100 patients with suspected cerebrovascular disease after injecting indium-111-labeled autologous platelets. One or more focuses of increased activity, implying local platelet accumulation, were seen along the course of the cervical carotid arteries in 52 patients. In 64 patients, there was a highly significant correlation between the results of scintigraphy and carotid arteriography (p = 10(6)). There was no significant correlation between the scintigraphic findings and the previous or subsequent occurrence of transient ischemic attack or cerebral infarction in the carotid circulation. These data suggest that factors other than the simple formation of platlet thrombi in the cervical carotid arteries are of primary importance in the pathogenesis of stroke.