This study aims to examine the relationship between sleep duration and body mass index (BMI) in two South Korean samples: children and adolescents. Nationally representative secondary data (i.e., the Korean Survey on the Obesity of Youth and Children) collected in 2009 were analyzed ("N" = 2,499 for children and "N" = 7,431 for adolescents). Descriptive statistics and analysis of variance were implemented to examine BMI levels depending on sleep duration. Multiple regression analysis was conducted to explore the existence of a strong association between sleep duration and BMI z score after controlling for other factors. In both samples, sleep length had a reverse linear relationship with BMI z score, and shorter sleep duration was significantly associated with an increase in BMI z score. School nurses should take note of the strong association between sleep duration and BMI for more effective weight control in children and adolescence. (Contains 3 tables and 2 figures.)