The phenotypic and genetic interrelationships underlying ADHD symptomatology assessed by various instruments were examined on a sample of 735 male and 819 female same-sex twin pairs, aged 8 to 16 years, participating in the first phase of the Virginia Twin Study of Adolescent Behavioral Development (VTSABD). Multivariate analyses were applied to parental and teacher ratings from an investigator-based interview, the CAPA, and three questionnaires (the CBCL and the Rutter Parent and Teacher Scales). Results from patterns of intercorrelations and factor analyses of maternal measures suggested that at the phenotypic level, these assessed the same underlying behavioural construct, which differed from other emotional and behavioural constructs. However, genetic analyses showed that in addition to a common factor underlying the expression of ADHD as assessed across the range of measures, additional genetic factors were identified that were method- and rater-specific. The findings suggest that although the investigator-based interview and the behavioural checklists tap similar aspects of ADHD behaviour, there is additional rater-specific variance.