The purpose of this study was to investigate the relationship of body mass index with environmental and dietary factors including nutrition knowledge, and nutrition attitude of Korean adolescents. Anthropometric measurements, questionnaire responses, and 24-hour dietary records of 531 elementary, 543 middle, and 533 high school students residing in Seoul and Kyunggi-do were collected. Data was analysed using SAS computer program. The results are as follows. Mean BMI of elementary school boys was higher than that of girls. However, there were no significant differences in mean BMI between sexes both in middle and high school students. Calcium and iron consumption were lowest among elementary and middle school students, respectively when the intake was expressed as a percentage of the Korean RDA. Energy, protein, vitamin A, vitamin B$_1$, vitamin B$_2$, niacin, and vitamin C intakes(% RDA) were lowest among high school students. The factors influencing BMI of elementary students were BMI of the parents in boys, and BMI of the father in girls. In middle school students, energy intake, fathers BMI, mothers nutrition knowledge and students BMI, and nutrition knowledge were related to the obesity indices of boys, while fathers and mothers BMI were related to the mean obesity index of girls. In high school students, nutrition knowledge and mothers nutrition attitude were related to the obesity index of boys, while BMI of parents was related to the obesity index of girls. In conclusion, mothers and fathers BMI were the most common factors influencing the BMI of these adolescents. Although these results can not determine if food behaviour directly affects BMI, it may influence the BMIs of family members, and therefore nutrition education to keep normal weight and desiradle food habits should be emphasized. More systematic studies to investigate the cause-effect relations of these factors are required.