Antibiotic prophylaxis is not routinely given for nonimplant, clean operations, although this view has recently been challenged. We have conducted a randomized multicenter, double-blind prospective trial to compare co-amoxiclav with placebo in 619 patients undergoing open groin hernia repair. Altogether 563 (91%) patients fulfilled the protocol; 283 received co-amoxiclav and 280 placebo. There was no difference between the groups in the number of patients receiving local or general anesthetic, the type of repair performed, the use of a subcutaneous fat suture, the type of skin closure used, the use of wound analgesia, or the use of a wound drain. Patients were given a card to return to the hospital in the event of their wound discharging or their needing to see their general practitioner. All patients were reviewed at approximately 6 weeks after operation. Fifty (8.9%) patients sustained a wound infection, 25 in the co-amoxiclav group and 25 in the placebo group. We conclude that antibiotic prophylaxis is of no benefit to patients undergoing open groin hernia repair.