Obesity is associated with an increased risk of many major chronic diseases. We estimated the economic costs of obesity-associated non-insulin-dependent diabetes mellitus, cardiovascular disease, gallbladder disease, cancer, and musculoskeletal disorders in 1990 US dollars, using a prevalence-based approach to cost-of-illness. In addition to direct costs, indirect costs were also estimated. The indirect cost of morbidity was estimated by calculating the costs associated with work days lost, and mortality costs were estimated on the basis of lifetime earnings lost. In 1990, the direct cost of obesity-associated disease in the US was $US45.8 billion, and the indirect cost of obesity was estimated to be $US23.0 billion. Therefore the total economic cost of obesity was estimated to be $US68.8 billion in 1990.