The circle of Willis has been demonstrated in the living human subject by serial angiography. The vessels were opacified by retrograde injection of contrast material through a cannula in the right brachial artery. The left common carotid artery was compressed, and the contrast material ascended through the right carotid and vertebral arteries. Roentgenograms were made in the submentovertical projection. The technique was used in more than 300 cases without mishaps. The results confirmed suspicions, based on anatomic studies, that communication between the anterior and posterior segments of the circle of Willis through the posterior communicating arteries is limited even under abnormal conditions in which adequate communication would reduce the damage from focal cerebral ischemia or cerebral infarct.