OBJECTIVE: To determine the clinical effectiveness of wound edge protection devices in reducing surgical site infection after abdominal surgery. DESIGN: Multicentre observer blinded randomised controlled trial. PARTICIPANTS: Patients undergoing laparotomy at 21 UK hospitals. INTERVENTIONS: Standard care or the use of a wound edge protection device during surgery. MAIN OUTCOME MEASURES: Surgical site infection within 30 days of surgery, assessed by blinded clinicians at seven and 30 days and by patient's self report for the intervening period. Secondary outcomes included quality of life, duration of stay in hospital, and the effect of characteristics of the patient and operation on the efficacy of the device. RESULTS: 760 patients were enrolled with 382 patients assigned to the device group and 378 to the control group. Six patients in the device group and five in the control group did not undergo laparotomy. Fourteen patients, seven in each group, were lost to follow-up. A total of 184 patients experienced surgical site infection within 30 days of surgery, 91/369 (24.7%) in the device group and 93/366 (25.4%) in the control group (odds ratio 0.97, 95% confidence interval 0.69 to 1.36; P=0.85). This lack of benefit was consistent across wound assessments performed by clinicians and those reported by patients and across all secondary outcomes. In the secondary analyses no subgroup could be identified in which there was evidence of clinical benefit associated with use of the device. CONCLUSIONS: Wound edge protection devices do not reduce the rate of surgical site infection in patients undergoing laparotomy, and therefore their routine use for this role cannot be recommended. TRIAL REGISTRATION: Current Controlled Trials ISRCTN 40402832.