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Evaluating the validity of self-reported smoking in Mexican adolescents. OBJECTIVES: We aimed to evaluate the validity of the self-reported smoking indicator used in the Global Youth Tobacco Survey (GYTS). SETTING: 43 middle and high-school classrooms from 26 schools were selected from Mexico City and Cuernavaca, Morelos. PARTICIPANTS: A total of 1257 students provided both a questionnaire and a urine sample. PRIMARY AND SECONDARY OUTCOME: Sensitivity and specificity of self-reported smoking compared to urinary cotinine. Validity indices were evaluated by subgroups of gender, social acceptability of smoking (ie, smoking parents or friends) and smoking frequency. RESULTS: Sensitivity and specificity for current smoking were 93.2% and 81.7%, respectively. Validity indices remained stable across gender. Parental smoking status moderated the validity of self-report, which had lower sensitivity in adolescents with non-smoking parents (86.7%) than in adolescents with smoking parents (96.6%). Sensitivity and specificity increased with smoking frequency. CONCLUSIONS: This first validation study of self-reported current smoking used in the GYTS among Mexican adolescents suggests that self-reported smoking in the past 30 days is a valid and stable indicator of current smoking behaviour. This measure appears suitable for public health research and surveillance.