The purpose of this study is to survey the prevalence of obesity assessed on the basis of height and weight among students in primary and secondary schools in the metropolitan area of Seoul, Korea during the period of 1979 to 1996. The major findings are as follows ; 1) The fiftieth percentile values of BMI have increased more in mid-ranged age group than upper(16-17 years old) and lower(6-7 years old) ranged age groups. 2) The prevalence of obesity by standard weight of height was higher among male students than female students and students in the primary schools showed higher weight increase than adolescent age(12-17 years old) group. As a whole, the prevalence of obesity over last 18 years has increased 4.6 times in males and 3.2 times in females. 3) Judged on the basis of the BMI 90th, 95th percentile values of 1979 , the prevalence of obesity among male students showed higher increases than among female students. Higher increase was also observed among primary school students than among middle and high school students. Increase of weight was higher for the $geq$95th percentile group than the 90-95th percentile group. 4) Judged on the basis of the BMI, 90th, 95 th percentile values of NHANES-I, the $geq$95th percentile group showed higher increase in the prevalence of obesity than the 90-95th percentile group. According to the 1996 data, severe obesity group ($geq$95th percentile) was about two times of NHANES-I in male primary school students, while the number for female middle and high school students was about 1/5 of NHANES-I.