OBJECTIVE: To acquire an understanding of the societal costs of dementia and how they affect families, health and social care services, and governments to improve the lives of people with dementia and their caregivers. METHODS: The basic design of this study was a societal, prevalence-based, gross cost-of-illness study in which costs were aggregated to World Health Organization regions and World Bank income groupings. RESULTS: The total estimated worldwide costs of dementia were US$604 billion in 2010. About 70% of the costs occurred in western Europe and North America. In such high-income regions, costs of informal care and the direct costs of social care contribute similar proportions of total costs, whereas the direct medical costs were much lower. In low- and middle-income countries, informal care accounts for the majority of total costs; direct social care costs are negligible. CONCLUSIONS: Worldwide costs of dementia are enormous and distributed inequitably. There is considerable potential for cost increases in coming years as the diagnosis and treatment gap is reduced. There is also likely to be a trend in low- and middle-income countries for social care costs to shift from the informal to the formal sector, with important implications for future aggregated costs and the financing of long-term care. Only by investing now in research and the development of cost-effective approaches to early diagnosis and care can future societal costs be anticipated and managed.