Background Extending endocrine therapy from 5 to 10 years is recommended for women with invasive estrogen receptor (ER)‐positive breast cancers. We evaluated the benefits and harms of the five additional years of therapy. Methods An established Cancer Intervention and Surveillance Network (CISNET) model used a lifetime horizon with national and clinical trial data on treatment efficacy and adverse events and other‐cause mortality among multiple birth cohorts of U.S. women ages 25–79 newly diagnosed with ER+, non‐metastatic breast cancer. We assumed 100% use of therapy. Outcomes included life years (LYs), quality‐adjusted life years (QALYs), and breast cancer mortality. Results were discounted at 3%. Sensitivity analyses tested a 15‐year time horizon and alternative assumptions. Results Extending tamoxifen therapy duration among women ages 25–49 reduced the lifetime probability of breast cancer death from 11.9% to 9.3% (absolute difference 2.6%). This translates to a gain of 0.77 LYs (281 days)/woman (undiscounted). Adverse events reduce this gain to 0.44 QALYs and after discounting, gains are 0.20 QALYs (73 days)/woman. Extended aromatase inhibitor therapy in women 50–79 had small absolute benefits and gains were offset by adverse events (loss of 0.06 discounted QALYs). There were greater gains with extended endocrine therapy for women with node‐positive versus negative cancers, but only women ages 25–49 and 50–59 had a net QALY gain. All gains were reduced with less than 100% treatment completion. Conclusion The extension of endocrine therapy from 5 to 10 years modestly improved lifetime breast cancer outcomes, but in some women, treatment‐related adverse events may outweigh benefits. Simulation modeling was used to evaluate extended endocrine therapy for women with hormone receptor‐positive non‐metastatic breast cancer. Results indicate that extension of endocrine therapy from 5 to 10 years modestly improves life years in some groups but adverse events may outweigh benefits.