The study is a post hoc analysis of the prospective controlled Swedish Obese Subjects (SOS) trial, in which 2,007 obese patients underwent bariatric surgery and 2,040 similar patients formed a control group without surgery. Patients were 37–60 years of age at enrollment and had a BMI ≥ 34 kg/m² in men and ≥ 38 kg/m² in women; median follow-up was approximately 27 years in the surgical and 25 years in the control group. The main outcome assessed was cancer incidence and cancer death, recorded from national registries. In women, bariatric surgery was associated with lower overall cancer incidence (adjusted hazard ratio HR = 0.78; 95% CI 0.67–0.90; p = 0.001) and lower overall cancer mortality (HR = 0.78; 95% CI 0.61–1.00; p = 0.050). In women, there was also a reduction in obesity-related cancers (HR = 0.70; 95% CI 0.58–0.85; p < 0.001) and women-specific cancers such as gynecological and breast cancers (HR = 0.60; 95% CI 0.47–…; p < 0.001). These favorable associations were strongest in women with higher baseline insulin levels, where a significant interaction between insulin and treatment was demonstrated (p = 0.021 for incidence and p = 0.039 for female-specific cancer mortality). In men, there was no reduction in overall cancer incidence or cancer mortality associated with bariatric surgery. The main limitation of the study is that cancer was not defined as the primary outcome in the original protocol.