Background.Some patients may be more predisposed to develop an ulceration of atherosclerotic plaque in the carotid artery, and emboli. These patients should be more at risk for a late stroke even after carotid endarterectomy than patients who are not.Materials and methods.Six-hundred thirty-eight patients had 750 carotid endarterectomies. Excised plaque specimens were examined for gross ulceration. The degree of stenosis was determined by duplex scan and/or angiography, and at operation. The median follow-up time was 3.6 years. The risk of a stroke occurring >30 days after carotid endarterectomy was calculated. Within 1 year, an endarterectomy of the contralateral artery was done in 77 patients (Subgroup) at a median time of 60 days.Results.Late stroke occurred in 48 patients. Patients who had had ulcerated plaque had a stroke at a median time of 2.0 years, and patients who had had no ulcers had a stroke at a median time of 5.2 years (P< 0.025). The 14-year stroke-free curve was lower (P< 0.05) if there was plaque ulceration. In the Subgroup, plaque ulcers were found in 55 patients (71%) at the first endarterectomy and later in 46 of the 55 patients (84%) in the contralateral artery. No ulcers were found in 22 patients (29%) initially, but later 50% had ulcers in the contralateral artery. The risk of an ulcer in the contralateral artery was increased (P< 0.005) in those patients who had ipsilateral carotid ulcers.Conclusions.Some patients appear to be predisposed for plaque ulceration and late stroke.