During the past decade, obesity has substantially increased in Korea, and this is leading to dramatic increases in complications such as type 2 diabetes. In this review, we discuss the past and the current situation of obesity in Korea based on the national health and nutrition surveys of 1995, 1998, and 2001. Because Korea is geographically isolated with relatively few migrants and has a low level of genetic heterogeneity, this report demonstrates the impact of environmental factors on the development of obesity beyond epidemiological information about one of the Asian countries. The third national health and nutrition survey reported in 2001 announced that the overall prevalence of obesity [body mass index (BMI) ≥ 25.0 kg m−2] in Korean adults is 30.6% (32.4% in men and 29.4% in women). The prevalence of obesity in adults and children has increased rapidly from 1990s through the beginning of the new millennium and is steadily increasing in parallel with our rapid socio-economic progress. In particular, special attention should be paid to two groups in hormonal transition: first, middle-aged men and women who experience a great increase in body weight, BMI and waist circumference because of andropause or menopause, and second, adolescents in middle to high school who are preparing for qualification or entrance examinations and are prone to overnutrition or nutritional imbalance and physical inactivity.