The authors used magnetic resonance angiography to examine the intracranial and cervical vascular structures of individuals who suffer from drop attacks. Normal structural configurations of the circle of Willis and the vertebrobasilar arterial system were compared to the vascular patterns of 10 subjects with these episodic tonic or atonic attacks. Overall, multiple areas of arterial occlusion, stenosis, or hypoplasia were visualized in the images of 8 of the 10 subjects. Specific anomalies of the vertebral and basilar arteries were identified in 4 individuals, and 8 images depicted nonvisualization of the posterior communicating arteries. We suggest that the pathological aberrations in the regional circulation of the hindbrain support the hypothesis that a transient hypovolemic episode may have an impact upon the neural activity involved in maintenance of motor tone and postural stability.