Sustained attention was studied in children with attention deficit hyperactivity disorder (ADHD) and normal controls using a continuous performance task with slow presentation of stimuli and carried out with an experimenter-present and absent condition. Children with ADHD were slower than controls, with performance deteriorating over time, particularly in the experimenter-absent condition. Both the slowness and deterioration normalized when the children received MPH. Hence the rate of stimuli presentation and the presence or absence of experimenter are both crucial factors in the performance of children with ADHD. MPH is able to ameliorate the performance decrement seen under these conditions.