BACKGROUND The aim of this study was to evaluate the cost-utility of bariatric surgery (gastric bypass, sleeve gastrectomy and gastric banding) compared with ordinary treatment in the Finnish healthcare system. METHODS Analysis was done from a healthcare provider's perspective using a combination of a decision tree and a Markov model, with a time horizon of 10 years. Health-related quality of life was estimated from a representative population survey, and other parameter values were based on registers, systematic reviews, controlled studies and expert opinion. RESULTS In the base-case analysis, bariatric surgery was both more effective and less costly than the ordinary treatment. The mean costs were €33,870 and €50,495, and the mean number of quality-adjusted life-years 7·63 and 7·05, for bariatric surgery and ordinary treatment respectively. Uncertainty around the parameter values was tested comprehensively in sensitivity analyses, and the results were robust. CONCLUSIONS Surgery for morbid obesity increases health-related quality of life, and reduces the need for further treatments and total healthcare costs. According to this analysis, non-operative care would be more costly for the Finnish healthcare system on average after 5 years following surgery.