Background. The impact of chemotherapy on cognition among elderly patients has received little attention, although such patients are more prone to presenting with age-related cognitive deficits and/or cognitive decline during chemotherapy. The present study assessed the cognitive function in older adults treated for early-stage breast cancer (EBC). Patients and Methods. The participants were newly diagnosed EBC patients aged $65 years without previous systemic treatment or neurological or psychiatric disease and matched healthy controls. They underwent two assessments: before starting adjuvant therapy and after the end of chemotherapy (including doxorubicin 6 docetaxel [CT1 group], n 5 58) or radiotherapy for patients who did not receive chemotherapy (CT2 group, n 5 61), and at the same interval for the healthy controls (n 5 62). Neuropsychological and geriatric assessments were performed. Neuropsychological data were analyzed using the Reliable Change Index. Results. Forty-nine percent of the patients (mean age, 70 6 4 years) had objective cognitive decline after adjuvant treatment that mainly concerned working memory. Among these patients, 64% developed a cognitive impairment after adjuvant treatment. Comorbidity was not associated with cognitive decline. No significant difference in objective cognitive decline was found between the two groups of patients; however, the CT1 group had more subjective cognitive complaints after treatment (p 5 .008). The oldest patients (aged 70–81 years) tended to have more objective decline with docetaxel (p 5 .05). Conclusion. This is the largest published study assessing cognitive function in older adults with EBC that included a group of patients treated with modern chemotherapy regimens. Approximately half the patients had objective cognitive decline after adjuvant treatment. The oldest patients were more likely to have cognitive decline with chemotherapy, particularly with docetaxel. The Oncologist 2016;21:1–12 Implications for Practice: This is the largest published study assessing cognitive function in older adults with early-stage breast cancer that included a group of patients treated with modern chemotherapy regimens. Approximately half the patients had objective cognitive decline after adjuvant treatment. The oldest patients were more likely to have cognitive decline with che-motherapy, particularly with docetaxel. Cognitive deficits could affect patients' quality of life and their compliance to treatment. Assessing cognitive dysfunctions in the elderly cancer population is a challenge in clinical practice, but it could influence the choice of the most appropriate therapy, including the use of oral drugs.