A neuropsychological investigation of the association between specific language impairment (SLI) and hyperactivity was conducted on four groups of 6-year-old children. The groups, each consisting of five boys and five girls, were: children with SLI alone, hyperactivity alone, both SLI and hyperactivity, and neither disorder (controls). Cognitive functioning was examined using selected non-verbal tests from the Cambridge Neuropsychological Test Automated Battery. SLI was not associated with significantly reduced performance on any of the neuropsychological measures. However, hyperactivity was associated with deficits on a test of attentional set shifting previously demonstrated to be sensitive to frontostriatal dysfunction. Further, hyperactive children showed significantly reduced spatial spans on a test of spatial working memory thought to measure parietal lobe functioning. There were no substantial differences between the groups on tests of frontal lobe function except for reduced performance on a task of attention in the hyperactive group. There were no significant interactions between hyperactivity and SLI on any of the neuropsychological measures. The pattern of neuropsychological deficits shown by children with attention-deficit-hyperactivity disorder (ADHD) suggests relatively specific problems in inhibitory control of attentional selection. This cognitive deficit implicates brain areas including basal ganglia and ventro-lateral prefrontal cortical areas in ADHD, thus providing evidence for disruption of frontostriatal functional loops. SLI and hyperactivity can occur as comorbid syndromes or symptom complexes and this should be noted when interpreting results of tests measuring cognitive performance. The findings suggest that these two groups of disorders have different cognitive correlates.