Objectives: The aim of the present study was to disentangle the impact of age and that of cancer diagnosis and treatment on functional status (FS) decline in older patients with cancer. Materials pod Methods: Patients with breast and colorectal cancer aged 50-69 years and aged >= 70 years who had undergone surgery, and older patients without cancer aged >= 70 years were included. FS was assessed at baseline and after 12 months follow-up, using the Katz index for activities of daily living (ADL) and the Lawton scale for instrumental activities of daily living (IADL). FS decline was defined as >= 1 point decrease on the ADL or IADL scale from baseline to 12 months follow-up. Results: In total, 179 older patients with cancer (>= 70 years), 341 younger patients with cancer (50-69 years) and 317 older patients without cancer (>= 70 years) were included. FS decline was found in 43.6%, 24.6% and 28.1% of the groups, respectively. FS decline was significantly worse in older compared to younger patients with cancer receiving no chemotherapy (44.5% versus 17.6%, p <0.001), but not for those who did receive chemotherapy (39.4% versus 30.8%, p = 0.33). Among the patients with cancer, FS decline was significantly associated with older age (OR 2.63), female sex (OR 3.72), colorectal cancer (OR 2.81), polypharmacy (OR 2.10) and, inversely, with baseline ADL dependency (OR 0.44). Conclusion: Cancer treatment, and older age are important predictors of FS decline. The relation of baseline ADL dependency and chemotherapy with FS decline suggest that the fittest of the older patients with cancer were selected for chemotherapy. (C) 2017 The Authors. Published by Elsevier Ltd.