BACKGROUND: Perioperative antibiotic prophylaxis to prevent surgical site infections (SSIs) after breast surgery is common practice. Breast SSIs were investigated to determine bacterial isolates, resistance patterns, and the appropriateness of cefazolin, the authors' institution's current regimen for perioperative antibiotic prophylaxis. METHODS: A retrospective review of 53 patients with culture-positive breast SSIs between June 1997 and August 2008 identified patient characteristics, bacterial isolates, and microbial resistance patterns. RESULTS: Among the 53 patients with positive cultures, 42% (n = 22) had undergone mastectomy, and 34% (n = 18) had undergone lumpectomy. Sixty-three bacterial isolates were identified, with 15% of SSIs being polymicrobial. Of the isolates, 49% (n = 31) were gram-negative bacteria. There was only 1 case of methicillin-resistant Staphylococcus aureus. Eight of 63 (13%) gram-negative isolates were cefazolin resistant. CONCLUSIONS: Gram-negative SSIs constituted half of the SSIs in this breast surgery cohort. Of all breast isolates, 17.5% were resistant to cefazolin. On the basis of these findings, antibiotic prophylaxis regimens alternative to cefazolin should be considered.