This study investigated the immediate effects of stimulant medication (methylphenidate) on cognitive attention processes in children with attention-deficit–hyperactivity disorder (ADHD). Thirteen males and two females (mean age 9y 5mo, SD 18.3mo) with a diagnosis of ADHD and who were to be prescribed methylphenidate were assessed twice on one day with the Test of Everyday Attention for Children, a neuropsychological battery designed to tap different aspects of cognitive attention. Between assessments, the children were administered methylphenidate (10mg). Each child had at least average intelligence (IQ 80 or over, as measured by the Wechsler Intelligence Scale for Children – III UK) and was on no other medication. A group of 16 children, who were matched for age, sex, and intelligence, also performed the cognitive tests twice on the same day to control for practice effects of testing. At the first assessment, children with ADHD demonstrated significant impairments in several aspects of cognitive attention in comparison with the control group, particularly sustained attention. After administration of methylphenidate for the children with ADHD, they showed significant improvements in their performance on measures of cognitive attention compared to controls. The immediate effects of methylphenidate and the significance of measuring cognitive aspects of attention as well as behavioural measures are discussed.