This study investigated EEG differences between children with Hyperkinetic Disorder (HKD), HKD sub-threshold attention deficit (HKDsub), and control children, in order to determine from an EEG perspective whether children with HKDsub represent a valid clinical disorder. Twenty-four boys were included in each of the three age-matched groups. The HKD group had greater total power and absolute delta and theta, more relative theta, and less relative alpha and beta than the control group. The HKDsub group had EEG profiles which were different from both control children and children with HKD, with the HKDsub group having EEG results generally between the HKD and control group. Additionally, a number of topographic differences were found in the frontal regions which suggest that the two HKD groups have independent EEG components. These results support the inclusion of a diagnostic category of attention deficit in future editions of the ICD.