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Background: Increasingly, patients with breast cancer undergo bilateral mastectomy (BLM). To the authors' knowledge, the magnitude of benefit is unknown.Methods: The authors used data from the Surveillance, Epidemiology, and End Results (SEER) program regarding all women diagnosed with American Joint Committee on Cancer stage 0 to stage III unilateral breast cancer in California from 1998 through 2015 and treated with BLM versus breast-conserving therapy including surgery and radiotherapy (BCT) or unilateral mastectomy (ULM). The authors measured relative risks of second contralateral breast cancer (CBC) and breast cancer death using Fine and Gray multivariable regression modeling adjusted for the competing risk of death and death from another cause, respectively, and potential confounding factors. Absolute excess risk of CBC was measured as the observed minus expected number of breast cancers in the general population divided by 10,000 person-years at risk.Results: Among 245,418 patients with a median follow-up of 6.7 years, 7784 patients (3.2%) developed CBC. Relative risks were lower after BLM (hazard ratio [HR], 0.10; 95% CI, 0.07-0.14) and higher after ULM (HR, 1.07; 95% CI, 1.02-1.13) versus BCT. Absolute excess risks were higher after BCT and ULM (5.0 and 13.6 more cases, respectively) compared with BLM (28.6 fewer cases). BLM reduced risk more among older women (38.0 fewer cases for women aged ≥50 years vs 17.9 fewer cases among women aged <50 years) but provided similar risk reduction across categories of tumor grade and tumor hormone receptor status. Compared with BCT, the risk of breast cancer death was equivalent after BLM (HR, 1.03; 95% CI, 0.96-1.11) and higher after ULM (HR, 1.21; 95% CI, 1.17-1.25).Conclusions: BLM may reduce second breast cancer risk by 34 to 43 cases per 10,000 person-years compared with other surgical procedures, but is not associated with a lower risk of death. Second breast cancers are rare, and their reduction should be weighed against the harms associated with BLM. [ABSTRACT FROM AUTHOR]