Objective: The purpose of this study was to ascertain the impact of obesity on the cost of disease management in people with or at high risk of atherothrombotic disease from a governmental perspective using a bottom-up approach to cost estimation. In addition, the aim was also to explore the causes of any differences found. Method: The health-care costs of obesity were estimated from 2819 participants recruited into the nationwide Australian REACH Registry with established atherothrombotic disease or at least three risk factors for atherothrombosis. Enrollment was in 2004, through primary care general practices. Information was collected on the use of cardiovascular drugs, hospitalizations and ambulatory care services. a??Bottom-upa?? costing was undertaken by assigning unit costs to each health-care item, based on Australian Government-reimbursed figures 2006a??2007. Linear-mixed models were used to estimate associations between direct medical costs and body mass index (BMI) categories.