Technical Abstract: This cross-sectional study tested the reliability and validity of the Block Kids Questionnaire to assess diet during the past 7 days. Within a 7-day period, 10- to 17-year-old children and adolescents completed two 24-hour dietary recalls by telephone, followed by the Block Kids Questionnaire at the end of the week. Test–retest reliability was assessed using intraclass correlations for 18 participants who completed a second Block Kids Questionnaire 1 month later. Validity of the Block Kids Questionnaire compared to the 24-hour dietary recall was assessed for the whole sample and by age group using paired t tests and Pearson correlation coefficients adjusted for attenuation and energy intake. Participants were 83 children and adolescents (57% Hispanic, 21% African-American, and 23% white; 53% were female subjects, mean age 13 years). The Block Kids Questionnaire mean daily consumption values were higher for percent energy from carbohydrate, and servings of fruit, 100% fruit juice, and vegetables, and lower for all other categories compared to the 24-hour dietary recall. All reliability intraclass correlations were >0.30, except percent energy from protein and fruit/vegetable servings. Significant differences in the means between the two dietary assessment methods were noted for most nutrients/food groups. The adjusted correlation coefficients ranged from 0.69 for percent energy from carbohydrate to –0.03 for grain servings, with 60% of the food group servings <0.18. Overall, the majority of the correlation coefficients for children aged >12 years were higher than those aged </=12 years. These results suggest that the Block Kids Questionnaire has validity for some nutrients, but not most food groups assessed, and appears more useful for adolescents.