INTRODUCTION: Older adults with cancer are exposed to greater difficulties in carrying out their daily activities due to cancer itself, its treatment, or both. The aim of this study was to describe functional decline after cancer diagnosis and to investigate the determinants of this decline among older individuals with cancer. MATERIALS AND METHODS: Using the Gironde cancer registries, older subjects (≥65 years) with a diagnosis of cancer between 2005 and 2018 were identified in three prospective cohorts on aging. Functional decline was defined as an increase of 1 point for Activities of Daily Living (ADL), 2 points for Instrumental Activities of Daily Living (IADL), and 3 points for the overall score (ADL + IADL) between cancer pre- and post-diagnosis visits. Logistic regression models were used to identify determinants of functional decline among older subjects who underwent a post-diagnostic assessment. Additionally, multinomial logistic regression models were performed to account for individuals who had died prior to the post-diagnostic cancer visit. RESULTS: A total of 306 individuals followed-up after the cancer diagnosis were included (median age at cancer diagnosis: 83; 44 % female). Older age at cancer, low educational level, impaired initial functional status, and poor five-year cancer-related prognosis were significantly associated with functional decline across all three scores. Multinomial logistic regression analyses (n = 489) yielded similar results, but only cancer-related factors, specifically unfavorable vital prognosis, were associated with higher risk of death. DISCUSSION: Functional decline in older individuals with cancer is both multifactorial and multidimensional. Further studies are needed to disentangle the effects of cancer and aging.< Réduire