The purpose of this study is to observe the longitudinal changes in BMI (Body Mass Index) of obese and non-obese 3rd. grade high school students in Seoul for 12 years and to see the trends of overt weight gain in obese adolescents. The results are as follows; 1. The average annual increasing rates of body mass Indices in male students were $1.14kg/m^2$ in obese group and $0.59kg/m^2$ in non-obese group. In female students, the average annual increasing rates of body mass indices were $0.93kg/m^2$ in obese group and $0.53kg/m^2$ in non-obese group. 2. The change rate of BMI for 12 years was significantly higher in obese group than non-obese group. 3. Puberty had less influence on the change rate of BMI in obese group compared to non-obese group. 4. In obese group, 71.8% of the variance in BMI at 17 can be predicted by BMI at 16 years in male students. In female students 44.4% can be predicted by BMI at age 16. 5. Among the 17-year-old obese students, 58.8% of the males and 56.2% of females were found not to have been obese at 7 years of age. 6. Among the 17-year-old obese students, those who were obese at 7 years of age were found to have higher BMI at later ages than those who were in the non-obese group. Obese adolescents were more likely to be obese in their childhood than non-obese group. There was no optimal age for the significant weight gain and the increasing rate of BMI was constantly higher in obese group than in non-obese group. Due to the fact that child obesity in early age contributes to obesity in adolescence, close observation is advised. On the other hand, a large proportion of obese adolescents can be preventable by early interventions, because about 50% of obese adolescents were not obese in early elementary school age.