IMPORTANCE: Bilateral mastectomy is increasingly used to treat unilateral breast cancer. Because it may have medical and psychosocial complications, a better understanding of its use and outcomes is essential to optimizing cancer care. OBJECTIVE: To compare use of and mortality after bilateral mastectomy, breast-conserving therapy with radiation, and unilateral mastectomy. DESIGN, SETTING, AND PARTICIPANTS: Observational cohort study within the population-based California Cancer Registry; participants were women diagnosed with stages 0-III unilateral breast cancer in California from 1998 through 2011, with median follow-up of 89.1 months. MAIN OUTCOMES AND MEASURES: Factors associated with surgery use (from polytomous logistic regression); overall and breast cancer–specific mortality (from propensity score weighting and Cox proportional hazards analysis). RESULTS: Among 189 734 patients, the rate of bilateral mastectomy increased from 2.0% (95% CI, 1.7%-2.2%) in 1998 to 12.3% (95% CI, 11.8%-12.9%) in 2011, an annual increase of 14.3% (95% CI, 13.1%-15.5%); among women younger than 40 years, the rate increased from 3.6% (95% CI, 2.3%-5.0%) in 1998 to 33% (95% CI, 29.8%-36.5%) in 2011. Bilateral mastectomy was more often used by non-Hispanic white women, those with private insurance, and those who received care at a National Cancer Institute (NCI)–designated cancer center (8.6% 95% CI, 8.1%-9.2% among NCI cancer center patients vs 6.0% 95% CI, 5.9%-6.1% among non-NCI cancer center patients; odds ratio OR, 1.13 95% CI, 1.04-1.22); in contrast, unilateral mastectomy was more often used by racial/ethnic minorities (Filipina, 52.8% 95% CI, 51.6%-54.0%; OR, 2.00 95% CI, 1.90-2.11 and Hispanic, 45.6% 95% CI, 45.0%-46.2%; OR, 1.16 95% CI, 1.13-1.20 vs non-Hispanic white, 35.2% 95% CI, 34.9%-35.5%) and those with public/Medicaid insurance (48.4% 95% CI, 47.8%-48.9%; OR, 1.08 95% CI, 1.05-1.11 vs private insurance, 36.6% 95% CI, 36.3%-36.8%). Compared with breast-conserving surgery with radiation (10-year mortality, 16.8% 95% CI, 16.6%-17.1%), unilateral mastectomy was associated with higher all-cause mortality (hazard ratio HR, 1.35 95% CI, 1.32-1.39; 10-year mortality, 20.1% 95% CI, 19.9%-20.4%). There was no significant mortality difference compared with bilateral mastectomy (HR, 1.02 95% CI, 0.94-1.11; 10-year mortality, 18.8% 95% CI, 18.6%-19.0%). Propensity analysis showed similar results. CONCLUSIONS AND RELEVANCE: Use of bilateral mastectomy increased significantly throughout California from 1998 through 2011 and was not associated with lower mortality than that achieved with breast-conserving surgery plus radiation. Unilateral mastectomy was associated with higher mortality than were the other 2 surgical options.