BACKGROUND: Group-based trajectory modeling is particularly important to identify subgroups of patients with pathological cognitive changes after cancer treatment. To date, only one study has explored cognitive trajectories in older patients with cancer. The present article describes objective cognitive changes before to after adjuvant treatment in older adults with early-stage breast cancer (EBC) after adjuvant treatment compared with healthy controls. PATIENTS AND METHODS: Participants were patients ≥65 years of age with newly diagnosed EBC and healthy controls (age-, sex-, and education-matched). The pretreatment assessment was conducted before adjuvant therapy, and the post-treatment assessment after the end of the first adjuvant treatment. Objective cognitive changes before to after treatment were evaluated based on the Reliable Change Index for cognitive decline accounting for cognitive impairment status. RESULTS: The sample consisted of women newly diagnosed with EBC (n = 118) and healthy controls (n = 62). Five patterns of changes before to after treatment were identified based on the presence of cognitive decline and cognitive impairment. The distribution of these five change patterns was statistically significant (p = .0001). Thirty-six percent of patients had phase shift changes, 31% without initial objective cognitive impairment developed impairment, 15% had a normal aging, 12% had a nonpathological decline, and 6% experienced accelerated cognitive decline. CONCLUSION: This study described for the first time objective cognitive changes before to after treatment of older adults with EBC immediately after the end of adjuvant treatment. A longer-term remote follow-up of adjuvant treatment is needed to better understand the cognitive trajectories of older patients with EBC. IMPLICATIONS FOR PRACTICE: After the end of adjuvant treatment, 31% of older adults with early-stage breast cancer without initial objective cognitive impairment developed impairment, and 6% experienced accelerated cognitive decline. Initial cognitive functioning should be included in the balance of benefits and harms of systemic therapy for patients who are likely to be at highest risk for cognitive decline after cancer treatments. Regular cognitive follow-up of patients who had cognitive impairment before cancer treatment should monitor symptoms suggestive of neurodegenerative disease and avert the effect of cognitive disorders on patients' autonomy.