AbstractBackgroundTo examine risks of cancers, obesity‐related cancers (eg, cancers in digestive organs, breast, ovary, kidney, thyroid, and myeloma), cancer‐related mortality, and all‐cause mortality in patients with type 2 diabetes mellitus (T2DM) and obesity who underwent bariatric surgery.MethodsA retrospective cohort of 1944 T2DM patients with obesity (345 bariatric surgery patients and 1599 matched controls) who were free of cancer from 2006 to 2017 was assembled. One‐to‐five propensity score matching followed by propensity score trimming was used to balance baseline covariates.ResultsDuring a mean follow‐up period of 37 months, there are risks that in 3.2%, 1.4%, 0.9%, and 3.2% of bariatric patients cancer, obesity‐related cancer, cancer‐related mortality, and all‐cause mortality, respectively, would occur. Surgical patients were found to have reduced incidence rates (IRs) of obesity‐related cancer (0.531/100 person‐years, 95% confidence interval [CI]: 0.172‐1.238/100 person‐years) and cancer of breast and genital organs (0.394/100 person‐years, 95% CI: 0.048‐1.424/100 person‐years) than matched control patients whose IRs for obesity‐related cancer and cancer of breast and genital organs were 0.627/100 person‐years (95% CI: 0.426‐0.889/100 person‐years) and 0.521/100 person‐years (95% CI: 0.277‐0.891/100 person‐years), respectively. Patients in the surgical group had a significant reduction in risk of all‐cause mortality (hazard ratio [HR] = 0.508, P = .041). Effects of bariatric surgery on any cancers (HR = 1.254, P = .510), obesity‐related cancers (HR = 0.843, P = .724), and cancer mortality (HR = 1.304, P = .694) were not significant.ConclusionsBariatric surgery was not associated with risks of overall cancer, obesity‐related cancer, and cancer mortality among T2DM patients with obesity at 3 years.