Abstract Background Consistent research findings indicate that parents and teachers observe genuinely different Attention Deficit/Hyperactivity Disorder (ADHD) behaviours in their respective settings. Objective To evaluate the utility of information provided by teacher informant assessments (INFAs) of ADHD symptoms, and the implications of aggregation algorithms in combing parents’ information, i.e. using ‘or-rule’ (endorsement by either one informant) versus ‘and-rule’ (endorsement by both informants). Method Teacher ratings on Conners scales and clinical data from parental accounts on 1383 probands and their siblings from the IMAGE study were analysed. The psychometric properties of teacher and combined ratings using the item response theory model (IRT) are presented. Kappa coefficients, intraclass correlations and linear regression were employed. Results First, teacher endorsement of symptoms is located in a narrow part of the trait continuum close to the average levels. Symptoms exhibit comparable perception in the measurement of the trait(s) with similar discrimination ability and information (reliability). Second, the IRT properties of the ‘or-rule’ ratings are predominantly influenced by parent-INFAs; and the ‘and-rule’ ratings predominantly by teacher-INFAs ratings. Third, parent-teacher INFAs agreement was low, both for individual items (κ = 0.01–0.15) and for dimensional scores (r = 0.12–0.16). The ‘or-rule’ captured milder expressions of ADHD symptoms, whereas the ‘and-rule’ indexed greater severity of ADHD. Conclusions Parent and teacher-INFAs provide different kinds of information, while both are useful. Teacher-INFA and the ‘and-rule’ provide a more accurate index of severity than an additive symptom count. Parent-INFA and the ‘or-rule’ are more sensitive for detecting cases with milder ADHD.