Background: Estimates of energy intake are required for an understanding of growth and disease; however, few methods of energy intake in children have been validated. Objective: Our objective was to validate energy intake estimated by the Youth-Adolescent Food-Frequency Questionnaire (YAQ) against the criterion total energy expenditure (TEE) by doubly labeled water (DLW). Design: Twenty-three boys and 27 girls (8.6-16.2 y of age) completed the YAQ and TEE measurements in 1 y. Results: Energy intake by the YAQ (10.03 ± 3.12 MJ) and energy expenditure by DLW (9.84 ± 1.79 MJ) were similar (P = 0.91) with large lower (Ϫ6.30 MJ) and upper (6.67 MJ) ± 2 SD limits of agreement. When within-subject CVs of repeated measures of the DLW and YAQ methods were used, 25 of the 50 subjects were deemed to have misreported their energy intake. The discrepancy in energy intake (YAQ -TEE) was related to body weight (r = Ϫ0.25, P = 0.077) and percentage body fat (r = Ϫ0.24, P = 0.09) but not to age (r = Ϫ0.07, P = 0.63) or the time between measures. From logistic regression, fatter boys were more likely to underreport energy intake than were fatter girls. Conclusion:The YAQ provides an accurate estimation of mean energy intake for a group but not for an individual.Am J Clin Nutr 2000;72:1455-60. KEY WORDSChildren, adolescents, youth-adolescent questionnaire, Youth-Adolescent Food-Frequency Questionnaire, food-frequency questionnaire, energy expenditure, energy intake, doubly labeled water INTRODUCTIONDietary intake influences normal growth, the development and progression of obesity, and many other conditions. Therefore, the ability to accurately measure energy and nutrient intake in individuals and populations is of great importance. Focus on nutritional intake in the first decades of life is particularly important because many lifelong nutritional habits may be established during childhood and adolescence (1, 2). Measurement of energy and nutrient intake in youngsters is challenging because of their lower literacy levels, cognitive and memory differences, knowledge deficits about food and food preparation techniques, and a general lack of interest in the subject matter (3).Baranowski and Domel (4) found that by age 10 y, most children are aware of the foods they have eaten and are able to give accurate information about their diet. Youth aged 9-18 y have shown the ability to complete a self-administered food-frequency questionnaire with reasonable consistency in responses over time (5). The accuracy of estimation of energy intake provided by food-frequency questionnaires may be assessed by comparison with doubly labeled water (DLW)-derived measurements of total energy expenditure (TEE). DLW-derived TEE is considered a criterion method for determining energy expenditure. Several studies showed that children and adolescents underreport their energy intake compared with their TEE (6-12). The subject's body composition, TEE, and age are significant factors in the magnitude of underreporting; fatter (6, 7, 10, 11) and older (6...