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Objective: To develop a food frequency questionnaire for adolescents (AFFQ) and demonstrate its relative validity. Design: The final version of the AFFQ was composed of 76 food items previously identified according to their contribution in nutrients and overall importance within the eating habits of this population group. The validation study, which was undertaken during a 6 month period (June to November 1999), was administered to a sample of 79 who answered at least three 24 h dietary recalls (R24 h) applied at intervals of 45 days and one AFFQ at the end of the study. Applying the paired t-test and calculating Pearson correlation coefficients on nutrient data, differences in the mean of nutrients were obtained. Correlation coefficients between the mean energy-adjusted nutrients computed by the two methods were calculated, and correction was made for within-person variability. Agreement was evaluated by distribution of the adolescents according to quartiles of consumption. Locus: A public school within the metropolitan region of São Paulo city. Results: A high variability in the dietary intake of adolescents was observed, with high rates of variability for cholesterol, retinal and vitamin C. The Pearson correlation coefficients, after being adjusted and corrected for variability, ranged from 0.10 to 0.72 among females and from 0.16 and 0.91 among males. The mean correlation coefficient for the entire group was 0.52. Conclusions: These results indicate that the AFFQ provides a potentially reliable scale for categorizing individuals by level of past intake of most nutrients, excluding retinol and iron. [ABSTRACT FROM AUTHOR]