Alzheimer's disease is characterised by a progressive loss of autonomy in activities of daily living. Many patients lose this autonomy rapidly with dramatic consequences for the patients and their relatives, and for health and social services. The aim of this study was to determine, in a large French cohort of community-living Alzheimer patients (REAL.FR), the factors underpinning different rates of loss of autonomy. Six hundred and eighty seven patients were recruited to this French cohort. Autonomy in activities of daily living was estimated with the IADL scale (Lawton). Patients were divided into three groups according to loss of autonomy during the first one-year follow-up period. Patients with a decrease in the IADL score had worse ADAS-cog score (Alzheimer's Disease Assessment Scale, cognitive subscale), Mini-Mental State Examination (MMSE), and Clinical Dementia Rating Scale scores. Up to a point, a marked decrease in the IADL score was less frequent among patients with a better ADAS-cog score (p < 0.10, bilateral test). At one-year evaluation, patients with a marked decline in autonomy were characterised by a greater decrease in Mini-Mental State score, faster progression of behavioural disturbance, and more hospitalisations. Further prospective studies, using established models, are needed to isolate the factors associated with a high rate of loss of autonomy in activities of daily living in Alzheimer patients.