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Objective: To evaluate therapy with aspirin plus dipyridamole in reducing restenosis after carotid endarterectomy.Patients: A total of 163 patients having 175 surgical carotid endarterectomies.Intervention: Eighty-three patients (90 endarterectomies) were randomly assigned to receive oral aspirin, 325 mg, plus dipyridamole, 75 mg, beginning 12 hours preoperatively, followed by a second dose administered within 8 hours after the operation, and given three times daily thereafter for 1 year. Eighty patients (85 endarterectomies) received placebo medication that was identical in appearance to the study drugs.Measurements: After the adequacy of the surgical procedure was confirmed by intraoperative angiography, restenosis at the endarterectomy sites was evaluated using serial duplex ultrasound studies before hospital discharge and at 3-month intervals postoperatively for 1 year.Results: Based on the time for developing identifiable restenosis and on efficacy analysis, greater than 50% restenosis developed in 11 operated vessels (16%) in the treated group and in 10 arteries (14%) in the placebo group, yielding an observed risk increase of 14% (95% CI, -52% to 167%; P greater than 0.2). By intention-to-treat analysis, greater than 50% restenosis developed in 16 of 90 operated vessels in treated patients and in 10 of 85 arteries in patients receiving placebo (26% for the treated group and 12% for the placebo group; P = 0.18, Mantel-Haenszel statistic), representing an observed risk increase of 110% (CI, -5% to 365%). Similar differences were observed for greater than 20% restenosis and for the comparison of patients rather than operated vessels by either intention-to-treat or efficacy analyses.Conclusions: Because therapy not only failed to reduce carotid restenosis but may have actually increased its frequency, treatment with aspirin plus dipyridamole probably has no clinically important benefit on restenosis in patients having carotid endarterectomy. [ABSTRACT FROM AUTHOR]