Background: Surgery has been used as part of breast cancer treatment for centuries; however any surgical procedure has the potential risk of infection. Infection rates for surgical treatment of breast cancer are documented at between three and 15%, higher than average for a clean surgical procedure. Pre and peri-operative antibiotics have been found to be useful in lowering infection rates in other surgical groups, yet there is no current consensus on prophylactic antibiotic use in breast cancer surgery. Objectives: To determine the effects of prophylactic antibiotics on the incidence of surgical site infection after breast cancer surgery. Search strategy: We searched the Cochrane Wounds Group Specialised Register (14 August 2008); The Cochrane Central Register of Controlled Trials (CENTRAL) - (The Cochrane Library Issue 3 2008); Ovid MEDLINE (1950 to August Week 1 2008); Ovid EMBASE (1980 to 2008 Week 32); and Ovid CINAHL (1982 to August Week 2 2008). No language restrictions were applied. Selection criteria: Randomised controlled trials of pre and peri-operative antibiotics for patients undergoing surgery for breast cancer were included. Primary outcomes were, incidence of breast wound infection and adverse reactions to treatment. Data collection and analysis: Two authors independently examined the title and abstracts of all studies identified by the search strategy, then assessed study quality and extracted data from those that met the inclusion criteria. Main results: For the first update of this review one additional trial was identified which met the inclusion criteria, a total of seven studies were included in the review. All seven evaluated pre-operative antibiotic compared with no antibiotic or placebo. Pooling of the results demonstrated that prophylactic antibiotics significantly reduce the incidence of surgical site infection for patients undergoing breast cancer surgery without reconstruction (pooled RR 0.72, 95% CI, 0.53 to 0.97). No studies presented separate data for patients who underwent reconstructive surgery at the time of removal of the breast tumour. Authors' conclusions: Prophylactic antibiotics reduce the risk of surgical site infection in patients undergoing surgery for breast cancer. The potential morbidity caused by infection, such as delays in wound healing or adjuvant cancer treatments must be balanced against the cost of treatment and potential adverse effects such as drug reactions or increased bacterial resistance. Further studies of patients undergoing immediate breast reconstruction would be useful as studies have identified this group as being at higher risk of infection than those who do not undergo immediate breast reconstruction.