The Healthy Eating Index (HEI) is a scoring system used by the US government to assess adherence to the Dietary Guidelines for Americans. We examined the ability of the HEI to monitor diet quality among youth. We modified and simplified the HEI for use by older children and adolescents. The new Youth Healthy Eating Index (YHEI) focuses on food quality and assesses both healthful and unhealthful foods and eating behaviors. Both HEI and YHEI scores were calculated from a food frequency questionnaire that was mailed to participants in the Growing Up Today Study in 1996. Girls (n=8,807) and boys (n=7,645) 9 to 14 years of age who are children of participants in the Nurses Health Study II cohort and who reside across the United States. Mean HEI and YHEI scores were calculated by sex and age, and associations with age, body mass index, activity, inactivity, energy intake, and several nutrients were assessed with Pearson correlations. Linear regression was used to examine the contributions of the individual HEI and YHEI components toward the total scores. The HEI score was highly correlated with total energy intake (r=0.67), indicating a strong association with quantity of food consumption. In contrast, the YHEI was not strongly correlated with energy intake (r=0.12) but was inversely associated with time spent in inactive pursuits (r=0.27). The HEI component for variety in food selection accounted for 60% of the variation in the total score and several HEI components were highly correlated with each other, particularly those for total and saturated fat (r=0.78). To successfully monitor diet in a population of children and adolescents, the HEI may benefit from modifications that focus on food quality and include assessments of unhealthful foods. Further research is needed to determine the dietary elements that are most related to health in diverse populations of youth.