Objective: To determine the attributable costs associated with surgical site infection (SSI) following breast surgery. Design and Setting: Cost analysis of a retrospective cohort in a tertiary care university hospital. Patients: All persons who underwent breast surgery other than breast-conserving surgery from July 1, 1999, through June 30, 2002. Main Outcome Measures: Surgical site infection and hospital costs. Costs included all those incurred in the original surgical admission and any readmission(s) within 1 year of surgery, inflation adjusted to US dollars in 2004. Results: Surgical site infection was identified in 50 women during the original surgical admission or at readmission to the hospital within 1 year of surgery (N=949). The incidence of SSI was 12.4% following mastectomy with immediate implant reconstruction, 6.2% following mastectomy with immediate reconstruction using a transverse rectus abdominis myocutaneous flap, 4.4% following mastectomy only, and 1.1% following breast reduction surgery. Of the SSI cases, 96.0% were identified at readmission to the hospital. Patients with SSI had crude median costs of $16 882 compared with $6123 for uninfected patients. After adjusting for the type of surgical procedure(s), breast cancer stage, and other variables associated with significantly increased costs using feasible generalized least squares, the attributable cost of SSI after breast surgery was $4091 (95% confidence interval, $2839-$5533). Conclusions: Surgical site infection after breast cancer surgical procedures was more common than expected for clean surgery and more common than SSI after non-cancer-related breast surgical procedures. Knowledge of the attributable costs of SSI in this patient population can be used to justify infection control interventions to reduce the risk of infection.