BACKGROUND AND OBJECTIVE: Collecting information from different raters is important for diagnosing ADHD, but several factors can lead to gathering discrepant information. Our aim was to determine the agreement between parent and teacher's when rating the list of ADHD symptoms (criterion A, DSM-IV) in a sample of Mexican school-age children. We explored whether inter-rater agreement varied by children's age and sex, and each symptom of inattention, hyperactivity, and impulsivity. METHODS: A total of 789 children (335 girls) from six elementary school grades grouped as G1 [grades 1-2], G2 [grades 3-4], and G3 [grades 5-6]) were rated by their parents and teachers. We identified inter-rater reliability by using Cohen's kappa coefficient by school level, sex, and ADHD symptoms. We explored the presence of symptoms considering parents' and teachers' ratings, individually and collapsed, using the AND/OR rules. RESULTS: Low inter-rater agreement was observed. Moderate levels were observed in G1, but not in G2 or G3. Both groups of informants reported that more boys than girls met these criteria, but agreement by sex was still low, as were the results of the analyses by individual symptoms. Among the children that met the ADHD criteria, an inattention symptom was the one most frequently reported by both raters, whereas among non-ADHD children, a hyperactive symptom was the one most often reported. DISCUSSION: The exclusive use of questionnaires fails to provide convergent information between raters. We highlight the importance of conducting comprehensive clinical histories when diagnosing ADHD in order to explore what these discrepancies show about the relationship symptoms/context.