Mastectomy is often recommended for women ≤40 years of age with breast cancer, as young women were under-represented in the landmark trials comparing breast conservation therapy (BCT) to mastectomy. We hypothesized that, in the modern treatment era, BCT and mastectomy result in equivalent local control rates in young women.Breast cancer cases arising between 1975 and 2013 in women ≤40 years old were collected from the tumor registries of 2 large healthcare systems in Utah. Kaplan-Meier estimates and Cox proportional hazards models were used to analyze freedom from locoregional recurrence (FFLR), overall survival (OS), and relapse-free survival (RFS).This analysis identified 853 BCT candidates. A comparison of BCT to mastectomy after 2000 showed FFLR, RFS, and OS were all similar. Rate for FFLR at 10 years was 94.9% versus 92.1% for BCT and mastectomy, respectively (P=.57). For women whose cancer was diagnosed after 2000, who received BCT, FFLR and RFS rates were improved compared to those whose cancer was diagnosed prior to 2000 (P<.05), whereas OS (P=.46) rates were similar. Among those who underwent mastectomy, FFLR, OS, and RFS were significantly improved (P<.05) with diagnosis after 2000.FFLR rates for young women, ≤40 years of age, have significantly improved for BCT and mastectomy over time. If patients were treated after 2000, BCT appears to be safe and equivalent to mastectomy at 10 years in terms of FFLR, OS, and RFS.