Background: Incisional herniation is a common complication of abdominal aortic aneurysm (AAA) repair. This study investigated whether prophylactic mesh placement could reduce the rate of postoperative incisional hernia after open repair of AAA. Methods: This randomized clinical trial was undertaken in three hospitals. Patients undergoing elective open AAA repair were randomized to routine abdominal mass closure after AAA repair or to prophylactic placement of polypropylene mesh in the preperitoneal plane. Results: Eighty-five patients with a mean age of 73 (range 59-89) years were recruited, 77 (91 per cent) of whom were men. There were five perioperative deaths (6 per cent), two in the control group and three in the mesh group (P = 0·663), none related to the mesh. Sixteen patients in the control group and five in the mesh group developed a postoperative incisional hernia (hazard ratio 4·10, 95 per cent confidence interval 1·72 to 9·82; P = 0·002). Hernias developed between 170 and 585 days after surgery in the control group, and between 336 and 1122 days in the mesh group. Four patients in the control group and one in the mesh group underwent incisional hernia repair (P = 0·375). No mesh became infected, but one was subsequently removed owing to seroma formation during laparotomy for small bowel obstruction. Conclusion: Mesh placement significantly reduced the rate of postoperative incisional hernia after open AAA repair without increasing the rate of complications. Registration number: ISRCTN28485581 (http://www.controlled-trials.com).