Introduction Despite initial increased rates of breast‐conserving therapy compared to mastectomy after 1990, mastectomy rates have increased in women under age 40 since 2000. Our study explores the demographic and survival implications of this trend. Methods The National Cancer Database was used to study stage 1 breast cancer diagnosed in women under age 40 between 2004 and 2014. Demographic and clinical data were obtained. Multivariable regression and survival analyses were performed. Results Of 11 859 patients under age 40, 57.2% underwent mastectomy (39.0% unilateral and 61.0% bilateral) rather than breast‐conserving therapy (42.8%). The rate of mastectomy was significantly higher in 2014 compared to 2004 (43.6% in 2004 vs 62.4% in 2014; P < 0.001). The rate of bilateral mastectomy was significantly higher in 2014 compared to 2004 in contrast to unilateral mastectomy (31.7% in 2004 vs 73.0% in 2014; P < 0.001). Non‐Hispanic Caucasian ethnicity and private insurance status were predictors of bilateral mastectomy (OR 2.06 [95% CI: 1.84‐2.30], P < 0.001; OR 1.39 [95% CI: 1.21‐1.59], P < 0.001). Controlling for demographics, tumor grade, and adjuvant therapies, bilateral mastectomy was associated with significantly increased 10‐year survival vs unilateral mastectomy (HR 0.75 [0.59‐0.96], P = 0.023). Additionally, breast‐conserving therapy was associated with significantly increased 10‐year survival vs unilateral (HR 2.36 [95% CI: 1.83‐3.05]; P < 0.001) and bilateral mastectomy (HR 2.30 [95% CI: 1.61‐3.27]; P < 0.001). Conclusions The majority of women under age 40 with stage 1 invasive breast cancer underwent mastectomy instead of breast‐conserving therapy. This largely reflects increased rates of contralateral prophylactic mastectomy. Bilateral mastectomy and breast‐conserving therapy vs unilateral mastectomy were associated with a small but significant increase in survival. This finding warrants further investigation to determine the clinical implications of decision‐making in younger women.

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