Design
Historical cohort study using Medicare claims data.

Setting
Medicare beneficiary population aged 65 years and older.

Patients
Medicare patients 65 years and older who received carotid endarterectomy during 1989.

Main Outcome Measures
Perioperative mortality rate and occurrence of transient ischemic attack, stroke, and death within 2 years of surgery.

Results
The risk for death within 2 years of the procedure was substantially higher among Medicare patients who received surgery in hospitals with perioperative mortality rates greater than rates demonstrated in the trials (odds ratio, 1.36; 95% confidence interval, 1.29-1.44) and among those with comorbidity that exceeded trial participation criteria (odds ratio, 1.72; 95% confidence interval, 1.64-1.84).

Conclusions
Although carotid endarterectomy trials demonstrate the efficacy of the procedure, this evidence only applies to patients and hospitals with characteristics comparable to those of the trial participants. The importance of this consideration is demonstrated by the substantially higher odds of death experienced among Medicare patients receiving the procedure who had a level of comorbidity that exceeded that of the trial participants or who received their surgery in a hospital with a perioperative mortality rate that exceeded the experience of the hospitals in the trials.