Objectives: The 2003 WHO/FAO technical report described that Korea has largely maintained its traditional high-vegetable diet despite major social and economic changes, and had lower than expected levels of obesity prevalence than other industrialized countries. However, the prevalence of obesity in Korea has recently been rapidly increasing. The aim of this study was to elucidate the determinants of this rapid growth of obesity prevalence in Korea and to compare the results of national nutrition surveys between Korea and Japan. Methods: The trends of the National Health and Nutrition Survey in Korea instituted every 3 years and that in Japan conducted every year were compared. The results of obesity prevalence defined as more than 25 of the Body Mass Index, the percentage of habitual exercisers and the results of the nutritional surveys were examined from 1992 to 2005 in Korea and from 1992 to 2004 in Japan. Results: The prevalence of obesity in males has been gradually increasing in both Korea and Japan since 1992. Though until 1995 the prevalence of obesity in the Korean male population was less than that in Japan, after 1998 Korea surpassed Japan and a markedly increasing trend was observed. In females, the increasing trend of obesity was slower than males in both Korea and Japan. However, the prevalence of obesity was much higher in Korea compared with that in Japan. The percentage of exercisers was much lower in Korea than in Japan. Although, the definition of an exerciser varies with the survey year in Korea and is different from that in Japan, almost 70% of the population was not regularly engaging in moderate or hard intensity exercise in Korea. From 1995, the total energy intake was increased by 9.8% in Korea but it was decreased by 6.9% in Japan. Presently, the energy intake per capita per day in Korea exceeded that in Japan. Remarkable increases in the intake of meat and poultry, vegetable oils and fats, and milk and dairy products were observed in Korea from 1995 to 2005. On the other hand, these values decreased during the same period in Japan. Conclusion: The prevalence of obesity in Korea is increasing and has surpassed that of Japan. The current trends could be attributed to the low prevalence of habitual exercisers, and an increase in energy intake and the proportion of energy intake from fat.