Although considerable research has focused on defining the precise nature of the behavioural symptomatology of ADHD: inattention, impulsivity, and hyperactivity (Swanson et al., 1998), the results are equivocal. This is partly because few studies have investigated different types of attention in the same ADHD population. Thus, the major goal of the current study was to further investigate the precise nature of ADHD 'inattention' symptomatology using two accepted measures of sustained and transient attention in the same population. Sustained attention performance was measured in 12 ADHD children and 25 normal control children using a traditional Continuous Performance Task (CPT) and a novel CPT. Transient attention performance was measured using a Visual Search Task (VST). The results revealed that the same group of ADHD children performed significantly worse on both the CPTs compared to the normals but that their performance on the VST was similar to that of normals. These results indicate that combined-type ADHD participants show impaired sustained attention, but not impaired transient attention performance, supporting the view that different ADHD subtypes have different deficit of attention.