We studied the prevalence and trend of obesity and metabolic syndrome in Korean adults aged at least 20 years using Korean National Health Examination and Nutrition Survey data from 1998 and 2001. The prevalence of body mass index (BMI) > or = 25 kg m(-2) among Korean adults in 2001 was 32.9% in men and 27.4% in women, and the number of Korean men with BMI > or = 25 kg m(-2) has increased markedly since 1998. A significant positive association between obesity and socioeconomic status was found in Korean men, whereas a significant negative association was identified in Korean women. The prevalence rates of waist circumference (WC) > or = 90 cm in men and WC > or = 85 cm in women were 23.4% and 23.1% in 2001 respectively. The prevalence of metabolic syndrome among Korean adults was 15-30% according to various criteria of metabolic syndrome. Future studies are needed to determine the changes in prevalence of obesity and contributing factors for obesity in Koreans.