This paper investigates the association between weight and elementary school students’ academic achievement, as measured by standardized Item Respond Theory scale scores in reading and math. Data for this study come from the 1998 cohort of the Early Childhood Longitudinal Study, Kindergarten-Fifth Grade (ECLS-K), which contains a large national sample of children between the ages of 5 and 12. Estimates of the association between weight and achievement were obtained by utilizing two regression model specifications, a mixed-effects linear model and a student-specific fixed-effects model. A comprehensive set of explanatory variables such as a household’s motivation in helping the student learn (e.g. parents’ expectations for their child’s schooling and levels of parental involvement with school activities), teacher qualification, and school characteristics are controlled for. The results show that malnourished children, both underweight and overweight, especially obese, achieve lower scores on standardized tests, particularly for mathematics, when compared to normal weight children. The outcomes are more pronounced for female students compared to male students. These results emphasize the need to reduce childhood malnutrition, especially childhood obesity.