PURPOSE: Treatment guidelines recommend multimodal therapy for non-metastatic high-risk breast cancer in older adults. However, older patients may be less likely to receive this due to varying abilities to withstand intensive therapy. We aimed to quantify the incidence of, factors associated with, and reasons behind omission of multimodal therapy in older high-risk breast cancer patients. METHODS: Women ≥ 70 years diagnosed with stage 2-3 HR-/HER2+ or triple-negative breast cancer were identified in the National Cancer Database, 2010-2020. Multimodal therapy was defined as surgery and systemic therapy; omission of multimodal therapy was defined as patients who did not receive one or both therapies. Chi-square tests were used to assess differences by therapy intensity. Multivariable logistic regression models adjusting for patient and disease-level characteristics were performed to determine the factors associated with therapy omission. RESULTS: Of 22,644 patients, 63.4% were ≤ 80 years old. Overall, 59.7% received multimodal therapy, 35.3% received either surgery or systemic therapy, and 5.0% received no therapy. Factors significantly associated with increased likelihood of multimodal therapy omission included increased age, Black race, Medicaid or uninsured status, and higher Charlson Comorbidity Index scores. The most common reason for omission was that it was "not part of planned treatment," (59.2% for omission of surgery, 52.4% for omission of systemic therapy), with patient refusal (17.4% for omission of surgery, 28.3% for omission of systemic therapy) being second most common. CONCLUSIONS: While most older patients received multimodal therapy, demographic and socioeconomic factors associated with treatment omission suggest that some vulnerable women with high-risk disease may be undertreated.