To assess the economic burden of Alzheimer's disease (AD), we carried out a cross-sectional prevalence cost-of-illness study in France. Fifty-one probable AD patients (NINCDS-ADRDA) actually treated in ambulatory care were recruited in two university outpatient centers. Demographic, clinical (including actual Mini-Mental State Examination scores), and economic data were collected by clinical investigators and trained interviewers. Total costs included actual expenditures such as direct medical costs and direct nonmedical costs, as well as indirect costs (loss of earnings due to loss of productivity). Cost valuation was based on the societal perspective using an opportunity costing approach. We found that indirect costs represented a significant portion of total costs (36%-40%). In terms of expenditures, patients and caregivers were found to bear the major part of AD total costs. We found a positive and significant correlation between disease severity and costs. Our findings support the hypothesis of a relationship between disease evolution and healthcare costs.