OBJECTIVES: This study aims to ascertain whether the differences of prevalence and severity of attention deficit hyperactivity disorder (ADHD) are true or whether children are perceived and rated differently by parent and teacher informant assessments (INFAs) according to gender, age, and co-occurring disorders, even at equal levels of latent ADHD traits. METHODS: Use of latent trait models (for binary responses) to evaluate measurement invariance in children with ADHD and their siblings from the International Multicenter ADHD Gene data. RESULTS: Substantial measurement noninvariance between parent and teacher INFAs was detected for seven out of nine inattention (IA) and six out of nine hyperactivity/impulsivity (HI) items; the correlations between parent and teacher INFAs for six IA and four HI items were not significantly different from zero, which suggests that parent and teacher INFAs are essentially rating different kinds of behaviours expressed in different settings, instead of measurement bias. However, age and gender did not affect substantially the endorsement probability of either IA or HI symptom criteria, regardless of INFA. For co-occurring disorders, teacher INFA ratings were largely unaffected by co-morbidity; conversely, parental endorsement of HI symptoms is substantially influenced by co-occurring oppositional defiant disorder. CONCLUSIONS: Our findings suggest general robustness of Diagnostic and Statistical Manual of Mental Disorders ADHD diagnostic items in relation to age and gender. Further research on classroom presentations is needed.