OBJECTIVE: The objective of this study was to test the hypothesis that bariatric surgery decreases the risk of esophageal and cardia adenocarcinoma. BACKGROUND: Obesity is strongly associated with esophageal adenocarcinoma and moderately with cardia adenocarcinoma, but whether weight loss prevents these tumors is unknown. METHODS: This population-based cohort study included patients with an obesity diagnosis in Sweden, Finland, or Denmark. Participants were divided into a bariatric surgery group and a nonoperated group. The incidence of esophageal and cardia adenocarcinoma (ECA) was first compared with the corresponding background population by calculating standardized incidence ratios (SIR) with 95% CIs. Second, the bariatric surgery group and the nonoperated group were compared using multivariable Cox regression, providing hazard ratios (HR) with 95% CI, adjusted for sex, age, comorbidity, calendar year, and country. RESULTS: Among 748,932 participants with an obesity diagnosis, 91,731 underwent bariatric surgery, predominantly gastric bypass (n=70,176; 76.5%). The SIRs of ECA decreased over time after gastric bypass, from SIR=2.2 (95% CI, 0.9-4.3) after 2 to 5 years to SIR=0.6 (95% CI, <0.1-3.6) after 10 to 40 years. Gastric bypass patients were also at a decreased risk of ECA compared with nonoperated patients with obesity [adjusted HR=0.6, 95% CI, 0.4-1.0 (0.98)], with decreasing point estimates over time. Gastric bypass was followed by a strongly decreased adjusted risk of esophageal adenocarcinoma (HR=0.3, 95% CI, 0.1-0.8) but not of cardia adenocarcinoma (HR=0.9, 95% CI, 0.5-1.6), when analyzed separately. There were no consistent associations between other bariatric procedures (mainly gastroplasty, gastric banding, sleeve gastrectomy, and biliopancreatic diversion) and ECA. CONCLUSIONS: Gastric bypass surgery may counteract the development of esophageal adenocarcinoma in morbidly obese individuals.