We identified mastectomy operations with at least one day follow-up after operation among women aged 18–64 years from 1/1/2004–12/31/2011 using ICD-9-CM and/or CPT-4 procedure codes from inpatient and outpatient facility and provider claims (Appendix 1). Because of coding inaccuracy and the limited clinical detail in claims data, we implemented steps to increase the likelihood that the procedures we included were truly mastectomies, as described below. We allowed a maximum of two mastectomies per woman during the study period. We excluded claims that contained CPT-4, HCPCS, or UB-04 revenue codes truncated to 4 digits and populated in the fields reserved for ICD-9-CM procedure codes and claims in which a mastectomy procedure code was present only on one line on a single claim with no other claims on the same date, as described previously.8 In 1,300 (6.7%) operations, CPT-4 or ICD-9-CM procedures codes for BCS were present during the same hospital admission or within 3 days of mastectomy. Since concurrent BCS and mastectomy is unlikely and the incidence of SSI after BCS is lower than after mastectomy, we created an algorithm to determine the most likely procedure. We included any of the following information as evidence that mastectomy was performed: procedure code for reconstruction (Appendix 1), CPT-4 pathology code 88309 (modified radical mastectomy), prophylactic removal of the breast (V50.41), mastectomy coded by both facility and surgeon, BCS and mastectomy on opposite breasts per CPT-4 modifier codes, BCS coded only by an assistant surgeon, or diagnosis of acquired absence of the breast in the year following surgery (V45.71). We excluded procedures more consistent with BCS, including surgeon coding only for BCS (mastectomy-only coded by assistant surgeon or facility), and other diagnoses and procedures consistent with BCS but not mastectomy (Appendix 2).9