BACKGROUND: Previous retrospective studies have shown that physical functioning in older cancer survivors is affected after treatment, yet prospective data are lacking. The aim of this study was to assess change in physical functioning in different age groups of patients with hormone receptor-positive breast cancer who were enrolled in the Tamoxifen Exemestane Adjuvant Multinational (TEAM) phase III trial. METHODS: Two physical parameters were assessed. Physical functioning was assessed using the European Organisation for Research and Treatment of Cancer (EORTC) QLQ-C30 questionnaire 1 year (T1) and 2 years (T2) after diagnosis. Physical activity was measured in metabolic equivalent of task (MET) hours/week at T1 and T2. Physical activity before diagnosis (T0) was assessed retrospectively at the T1 questionnaire. Patients were divided into three age groups: <60, 60-69, and ≥70 years. Decline in physical functioning was assessed using linear regression analysis. Differences in mean values of physical activity levels were calculated using repeated-measures one-way analysis of variance. RESULTS: A total of 431 patients were included for analysis. In all age groups, physical activity levels at T1 and T2 were significantly lower than prediagnostic physical activity levels (T0) (p < .001 for all age groups). Age ≥70 years was independently associated with decline in physical functioning between T1 and T2 (β = -4.62, 95% confidence interval -8.73 to -0.51, p = .028). CONCLUSION: Patients aged 70 years or older treated with breast surgery and adjuvant hormonal therapy did not improve between years 1 and 2 after diagnosis to the same extent as did younger patients. IMPLICATIONS FOR PRACTICE: Although older patients constitute a large share of the breast cancer population, little is known about the effect and consequences of treatment of breast cancer in this specific age group. This study revealed that, unlike younger patients, older patients do not regain their physical abilities after surgical and adjuvant treatment for breast cancer. In older adults, the effect of treatment on physical functioning and independency could be more relevant than survival outcomes. Clinicians and older patients should be aware of the impact of treatment on physical functioning and prevent older patients from experiencing physical decline, which could lead to institutionalization and loss of independence. There is a need for age-specific guidelines that take into account the heterogeneity of the older population and for evidence-based treatment that focuses not only on cancer-specific outcomes but also on the consequences of treatment for physical and cognitive functioning and quality of life.