The risk of new-onset atrial fibrillation (AF) is reduced by weight loss after bariatric surgery. This finding comes from the SOS study conducted in Sweden. The prospective, matched cohort study involved 4,021 obese individuals in sinus rhythm and with no history of AF; 2,000 underwent bariatric surgery, and 2,021 received usual care. Bariatric surgery was associated with sustained weight reduction (18% weight loss after 20 years), whereas weight remained largely unchanged in the usual-care group. During the 19-year follow-up, new-onset AF occurred in 12.4% of the surgical group and 16.8% of the control group (HR 0.71, 95% CI 0.60–0.83, P <0.001). The risk reduction was greatest in young individuals and those with high diastolic blood pressure.