As research on impulsive behaviour in hyperactive children has been rather neglected, this study first measured impulsivity using several instruments and then compared the outcomes. 31 boys with ADHD participated in this study as well as an equal sized male control group (mean age 9;4 years). Each subject completed two computerised tests of impulsivity: the Delay of Gratification Test (DGT) as well as the Continuous Performance Test (CPT). Additionally, the boy's impulsive behaviour was rated by two questionnaires for teachers (the Teacher Report Form, TRF, and the Conners Teacher Rating Scale, CTRS). Apart from impulsive behaviour the motor activity was measured, as the classification systems ICD-10 and DSM-IV differ on the concepts of "impulsivity" and "hyperactivity". The assessment was undertaken using a radar-actometer in order to find an association between "impulsivity" and "hyperactivity". As expected, there was a significant difference between the two groups concerning impulsive behaviour and motor activity. As opposed to that result, the correlations of the several instruments that measure impulsive behaviour were not constantly significant. The impulsivity-item on the CTRS is not associated with the CPT and correlates moderate significantly with the DGT. These results led to the development of a model that includes the construct of "impulsivity" as separate dimension of ADHD.