Background: There is little longitudinal data on resource use and costs associated with Alzheimer's disease (AD) in France.; Objectives: To evaluate resource use and societal costs associated with AD in a French cohort of patients and their caregivers and the effect of patient cognitive decline on costs over an 18-month period.; Methods: Community-dwelling patients with mild, moderate, or moderately severe/severe AD dementia (n = 419) were followed-up for 18 months. Total societal costs were estimated by applying 2010 unit costs to resource use, including outpatient visits, hospital days, institutionalization, and caregiver hours. Cognitive function was assessed by Mini-Mental State Examination scores.; Results: Mean cumulative total costs over the 18-month period were €24,140 for patients with mild AD dementia, €34,287 for those with moderate AD dementia, and €44,171 for those with moderately severe/severe AD dementia (P < 0.001; ANOVA comparison between severity groups). The biggest contributor to total societal costs was caregiver informal care (>50% of total costs at all stages of AD dementia). Cognitive decline (≥3-point decrease in Mini-Mental State Examination score or institutionalization) was associated with a 12.5% increase in total costs (P = 0.02). Significant differences were observed across severity groups for caregiver time (P < 0.001); mean monthly caregiver time increased at each time point over the 18 months in each severity group.; Conclusions: Increasing severity of AD dementia in France is associated with increased use of resources as well as increased total societal and patient costs; informal care was the greatest cost contributor. Clinically meaningful cognitive decline is associated with significantly increased costs.