The rising trend in the prevalence of obesity has become a major public health concern in many countries during the past decades, partly because being obese is associated with comorbidities and death. The cost of treatment for obesity related diseases has become a heavy burden on the national health care budget in many countries. The treatment options for obesity are mainly weight management therapies in the form of diet and exercise, pharmacological therapy, and surgery. The incidence of

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... ric surgery in Sweden has increased eight-fold in the last decade. Our objective was to assess the cost-effectiveness of gastric bypass surgical treatments for obesity in adult patients, in comparison with conventional treatment, in Sweden from a societal perspective. The conventional treatment alternative consisted of the prevalent mixture of non-surgical obesity treatments. A model of individual patients was used to simulate the outcomes of the patients in terms of treatment costs, indirect costs, life years, and quality adjusted life years (QALY) over a lifetime perspective. In patients with a Body Mass Index of 40-44 kg/m 2 , surgery was estimated to be cost-saving in men and judged cost-effective in women, with an incremental cost per QALY gained of SEK 26 thousand (EUR 3 thousand). The incremental cost associated with gastric bypass decreases with BMI, increases with the patient's age, and is higher in women than in men. Taking patient characteristics and uncertainty in input data and model design into account, the incremental cost is estimated to be at most SEK 160 thousand per QALY gained (about EUR 18 thousand per QALY). In conclusion, gastric bypass surgery appears to be a cost-effective intervention compared to conventional treatment in adult persons with obesity in Sweden.