During 20 years (1957-1977), 1286 carotid endarterectomies were performed on 1022 private patients with cerebrovascular insufficiency. Included were 132 patients undergoing 167 endarterectomies for asymptomatic cervical carotid bruits. Ages ranged from 42 to 82 years (mean: 64.7). Operative mortality was zero. There were two transient and two permanent operation-related neurologic deficits. Complete follow-up was achieved, extending to 184 months. During postoperative follow-up, six patients (4.5%) developed TIA's appropriate to the unoperated artery, three patients had strokes (2.3%), and three patients died of strokes (2.3%). To characterize the natural history of asymptomatic bruit and determine proper indications for prophylactic endarterectomy, a control series of 138 additional patients with asymptomatic bruit not operated upon when the bruit was discovered was studied. Ages ranged from 39 to 86 years (mean: 65.7). During follow-up extending to 180 months, 77 patients (55.8%) remained neurologically asymptomatic, 37 patients (26.8%) developed TIA's one month to 99 months after detection of bruit, and 24 patients (17.4%) sustained mild to profound frank strokes one week to 124 months postdetection. Three of these 24 (2.2%) died of stroke. Asymptomatic carotid bruits may be potential stroke hazards, the risk of which can be significantly reduced by appropriately applied endarterectomy. A protocol for managment is presented.