To determine the natural history and durability of bilateral carotid endarterectomy (CEA), we studied 27 patients who participated in a randomized, prospective, consecutive entry trial comparing vein patch with primary CEA closure. This cohort represented 20% of the 136 patients who took part in the 4-year study. Bilateral CEAs were planned at the time of the original admission in 13 (48%), whereas 14 (52%) developed late indications for contralateral CEA a mean of 27 +/- 7 months after the initial procedure. Among the 27 patients, 15 underwent alternating methods of CEA closure. During a mean follow-up of 64 +/- 7 months, five patients had six recurrences (four unilateral, one bilateral). The type of closure did not affect the recurrence rate. All recurrences were asymptomatic and measured < 50% diameter loss by duplex criteria. There were no strokes. Two other patients had late transient ischemic attacks, but neither of them had arteriographic evidence of recurrent carotid disease. No patient underwent reoperative CEA. These data demonstrate that bilateral CEA is durable. Late recurrences are rare and clinically insignificant. The higher rate of unilateral recurrence suggests that local factors play a more important role than systemic factors in the etiology of recurrent disease.