Validity and reliability of the quality of masticatory function questionnaire applied in Brazilian adolescents.  Purpose proper chewing and swallowing functions have great importance in general health, since it potentially affects food''s digestion and the subject''s nutritional status. The aim was to assess the validity and reliability of the self-applied Quality of Masticatory Function Questionnaire (QMFQ) in a convenience (non-referred) sample of Brazilian adolescents divided into three groups: control (n=57), dental caries (n=51) and malocclusion (n=54). Methods caries and malocclusion were evaluated using DMF-S index (number of decayed, missing and filled primary and permanent surfaces) and Index of Orthodontic Treatment Need, respectively. The QMFQ comprises 26 items regarding frequency and intensity of chewing problems with five domains: Food-Mastication, Habits, Meat, Fruits and Vegetables. Data were submitted to descriptive statistics, Kolmogorov-Smirnov and Chi-square tests. Psychometric evaluation included measures of reliability (internal consistency - Cronbach''s alpha and matrix of correlation) and discriminant validity (Kruskal-Wallis/Dunn post-test). Results the instrument showed satisfactory internal consistency, with significant positive correlations between the domains scores except between Habits and Vegetables. Total scale Cronbach''s alpha coefficient was 0.87 and the coefficient did not increase significantly with the removal of each domain. Significant differences were found between controls and caries group in Food-Mastication, Meat and Fruits scores. Caries group also showed higher median values in Food-Mastication and Fruits than the malocclusion group. Conclusion the Quality of Masticatory Function Questionnaire showed acceptable properties regarding internal consistency, reliability, and discriminant validity in evaluating the impact of caries on the perception of the masticatory function quality of Brazilian adolescents.