The Swedish Obese Subjects (SOS) Study was conducted to determine whether obese patients actually can improve their mortality risk by losing weight. After an initial screening study, a total of 6328 subjects were recruited and comprehensively characterized. In the following interventional study, 2010 of the subjects underwent surgery for obesity (gastric banding, gastroplasty, gastric bypass) while 2037 chose a conventional form of treatment, and acted as controls (matched pairs). After 10 years, the controls had gained an average of 1.4 kg in weight. The surgical subjects, in contrast, showed an appreciable and persisting decrease in weight. In comparison with the control group, this group showed a clear decrease in the incidence of cardiovascular risk factors (among others: hypertension, hypertriglyceridemia, diabetes) as well as an improvement in cardiac function parameters and health-related quality of life. On the basis of the present data, however, no definitive pronouncement on the relationship between weight reduction and decrease in overall mortality rate can yet be made.