There is still considerable uncertainty regarding the prognosis and management of patients with intermittent circulatory insufficiency of those cerebral and brain-stem regions normally supplied by the vertebral-basilar system. Currently popular views recommend either surgical intervention1,2or administration of anticoagulant drugs for varying periods.3,4Although prognosis, according to the proponents of either of these therapeutic modalities, is reported to be quite good, one must question whether prognosis has actually been improved by attempts at active therapy, since there is still a paucity of information regarding the clinical course of untreated patients. This uncertainty may well be compounded by the lack of quantitative means of estimating the hemodynamic consequences of vertebral-basilar stenosis or occlusion. In the present study, 26 patients whose symptoms were clinically characteristic of vertebral-basilar insufficiency were evaluated from the standpoint of clinical course, over-all cerebral hemodynamic measurements, and influence of anticoagulants. Method The 26 subjects of the