OBJECTIVE: The study aims to (1) estimate the costs of hospital treatmetn and the value of lost production due to early death associated with underweight, overweight and obese patients, and to extrapolate the findings to national costs and (2) estimate the potential benefits of an obesity intervention program in terms of cost-offsets. METHODS: For the first aim we use a retrospective analysis of the hospital treatment episodes of a defined population with data obtained from screening of 33,196 middle-aged subjects living in Malmo, Sweden and collected during a 15-year follow-up period. For the second aim we apply a prospective cost analysis within a modeling framework. RESULTS: The yearly excess hospital (somatic, psychiatric) care cost (SEK) for overweight (25-BMI-29 kg/sqm) and obesity (BMI 30 kg/sqm and above) was estimated to SEK 1300 million (US$130 million, assuming $1=SEK10), or about 2.3 percent of total hospital care costs in Sweden. Indirect costs due to early mortality for obese subjects were estimated to SEK 1200 million (US$120 million). For males at age 55+ the potential hospital costs savings, excluding costs of intervention, that could be gained by an intervention that successfully and sefely could alter the weight of an overweight or obese individual to become normal weight was estimated to SEK 30,000 (US$3,000) over 15 years. CONCLUSION: Hospital treatment costs are found to have a J-shaped relationship to weight (BMI) indicating potential cost offsets by effective weight-loss intervention.