PURPOSE: The aim of the present study was to assess, by using a cost-benefit analysis, the net monetary benefit (NMB) of bariatric surgery compared with diet (including physical exercise) for obese patients, from both an Italian payer perspective and the broader societal perspective. METHODS: The study considered the following groups of patients: (1) patients with a body mass index (BMI) ≥40 kg/m2 without complications + patients with BMI ≥35 kg/m2 with complications; (2) patients with BMI ≥35 kg/m2 and diabetes; and (3) patients with BMI ranging from 30 to 35 kg/m2 and diabetes. A Markov model was developed to project the lifetime health outcomes (life years and quality-adjusted life years [QALYs]) and costs associated with bariatric surgery and diet for the considered groups of patients. The clinical effectiveness of each strategy was based on the likelihood of experiencing cardiovascular events or events related to the presence of diabetes. Data on clinical effectiveness, quality of life, productivity losses, and out-of-pocket costs were mainly derived from the literature; direct costs were obtained from official tariffs or the literature. Different scenarios were considered for the analyses in addition to the base case. According to both perspectives considered, the NMB was calculated by first assuming a willingness-to-pay threshold (30,000€ per QALY), then converting health benefits (QALYs) into the common monetary metric (ie, the euro). NMB was calculated as follows: (incremental benefit × willingness-to-pay - incremental cost). FINDINGS: For all the scenarios and groups of patients considered, the NMB of bariatric surgery versus diet, on a lifetime horizon, from the payer perspective was positive and ranged from 54,647€ to 122,960€; it varied between 141,192€ and 380,286€ from the societal perspective. In the former case, the NMB turns positive after 3-4 years, indicating that bariatric surgery may be a worthy investment also in the short run for the National Health Service; in the latter case, for a time horizon longer than 2-3 years, the surgical option begins to show advantages for the whole society. IMPLICATIONS: Despite its defined cost-effectiveness, bariatric surgery is under-diffused because the initial investment for the technology is often considered a barrier. The cost-benefit analysis showed that bariatric surgery, compared with diet, may be a worthwhile technology for obese patients in Italy from both a payer perspective and the broader societal perspective.