We examined trends in obesity among Korean adults, using body mass index (BMI) and waist circumference (WC) as reported in national surveys. Data (10,043 men and 12,758 non-pregnant women) were derived from four waves of the Korea National Health and Nutrition Examination Survey conducted in 1998, 2001, 2005, and 2007. Between 1998 and 2007, the distribution of BMI and WC showed shifts toward the right among men. Mean values of BMI and WC and the corresponding overweight (includes obesity) and obesity prevalences showed increasing trends in men but not in women. Women aged 60+ showed significant increases in obesity measures, including mean BMI and WC, and the associated prevalences. Among women aged 20-39, the prevalence of underweight increased significantly between 1998 and 2007, and BMI showed a decreasing tendency. These time trends in young women were the reverse of the trends in young men. In conclusion, policy efforts to abate overweight and obesity trends need to be exercised among men and older women. In addition, more national studies regarding potential increases in underweight among young women are warranted.