BACKGROUND: Older women with breast cancer frequently experience toxicity-related hospitalizations during adjuvant chemotherapy. Although the geriatric assessment can identify those at risk, its use in clinic remains limited. One simple, low-cost marker of vulnerability in older persons is fall history. Here, the authors examined whether falls prechemotherapy can identify older women at risk for toxicity-related hospitalization during adjuvant chemotherapy for breast cancer. METHODS: In a prospective study of women >65 years old with stage I-III breast cancer treated with adjuvant chemotherapy, the authors assessed baseline falls in the past 6 months as a categorical variable: no fall, one fall, and more than one fall. The primary end point was incident hospitalization during chemotherapy attributable to toxicity. Multivariable logistic regression was used to examine the association between falls and toxicity-related hospitalization, adjusting for sociodemographic, disease, and geriatric covariates. RESULTS: Of the 497 participants, 60 (12.1%) reported falling before chemotherapy, and 114 (22.9%) had one or more toxicity-related hospitalizations. After adjusting for sociodemographic, disease, and geriatric characteristics, women who fell more than once within 6 months before chemotherapy had greater odds of being hospitalized from toxicity during chemotherapy compared to women who did not fall (50.0% vs. 20.8% experienced toxicity-related hospitalization, odds ratio, 4.38; 95% confidence interval, 1.66-11.54, p = .003). CONCLUSIONS: In this cohort of older women with early breast cancer, women who experienced more than one fall before chemotherapy had an over 4-fold increased risk of toxicity-related hospitalization during chemotherapy, independent of sociodemographic, disease, and geriatric factors.