PURPOSE: In Korea, the results of bariatric surgery have not been compared with those of nonsurgical treatment. The purpose of this study was to evaluate the effectiveness and safety of bariatric surgery vs. conventional nonsurgical treatment in severely obese Koreans. METHODS: In this retrospective cohort study, we reviewed the medical charts of 261 consecutive subjects who underwent bariatric surgery and 224 subjects who were treated with weight control medication and lifestyle modification therapy between January 2008 and February 2011. Measures of clinical effectiveness, including change in weight (%) and comorbid diseases, and occurrence of complications, were investigated for 18 months after bariatric surgery. RESULTS: Body mass index (BMI) was higher in the surgery group than in the conventionally treated group (mean +/- standard deviation, 39.0 +/- 6.2 vs. 34.3 +/- 3.8). Diabetes was more prevalent in the surgery group than in the conventionally treated group (39.1% vs. 12.9%). The change in weight (%) between baseline and 18 months posttreatment was significantly greater in the surgery group (22.6%) than in the conventional therapy group (6.7%). While 57%, 47%, and 84% of subjects recovered from diabetes, hypertension, and dyslipidemia, respectively, in the surgery group, 10%, 20%, and 24% of subjects recovered from these conditions in the conventional group. Fifty-one subjects (19.5%) in the surgery group reported 61 complications (23.4%). CONCLUSION: Bariatric surgery in Korea was significantly more effective than conventional treatment for weight loss and recovery from comorbidities such as diabetes, hypertension, and dyslipidemia, with a reasonable complication rate.