A review of more than 5,000 cerebral angiograms revealed circular spastic contractions of the cervical carotid artery in 5 per cent of the cases. In addition, 28 examples of other nonatheromatous irregularities and dilatations of that vessel were found. Sixteen of these were fibromuscular dysplasia, 8 were aneurysms, 2 were arteriovenous fistulas, and 2 were stationary arterial waves. The characteristic angiographic finding with circular spastic contractions was narrowing of the lumen in the midportion of the internal carotid artery. The origin of this vessel was frequently involved as well. Two distinct forms of constriction could be recognized: (1) sharply localized and bandlike in character; and (2) more diffuse and tubular. Most often some combination of the two forms was present. The changes, in most cases, resulted from excessive manipulation during cannulation of the artery or subintimal or perivascular injection. Minor narrowing of the arterial lumen, which had a monotonously regular, corrugated appearance and affected the midportion of the artery, was characteristic of stationary arterial waves. Related slowing of the intracranial circulation on the ipsilateral side was present in both cases. Typical cases of fibromuscular dysplasia featured alternating zones of widening and narrowing of the arterial lumen. Distinguishing features included dilatations which were always wider than the normal lumen and were separated from one another by sharply localized constrictions. The proximal 2 to 2.5 cm. of the internal carotid artery was never involved. Atypical cases of fibromuscular dysplasia had similar features except that one wall of the artery in the involved segment was unaffected. Aneurysms and arteriovenous fistulas always showed saccular or fusiform widenings of the vascular channels. Recognition of the venous component in the fistulas distinguished the two conditions.