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To study sustained performance and its relation to regulation of effort among four different groups of children: two clinical subgroups of children diagnosed with attention-deficit hyperactivity disorder (ADHD) – one with and one without comorbid oppositional defiant disorder (ODD); a control sample of children without a diagnosis of ADHD but with parental reports of hyperactivity; and a normal control sample.Sustained performance was studied (n = 64) using a choice reaction time task with long interstimulus intervals, and hyperactive behaviour and regulation of effort was rated throughout the task.The clinical sample differed from the normal controls regarding all measures of overall, but not initial, performance. When studying performance over time, the ADHD group was shown to differ from the normal controls with regard to omissions, mean reaction time and regulation of effort. No significant differences were found between the ADHD children with or without comorbid ODD. The non-clinical hyperactive children differed from the normal controls with regard to reaction time, initially and overall, and regulation of effort overall.Deficits in sustained performance among ADHD children appear to be evident when long interstimulus intervals are used, and these deficits are related to poor regulation of effort. Further, the observed differences between ADHD children and normal controls do not appear to be a result of comorbid conduct problems. The non-clinical hyperactive children were more similar to the control group than to the ADHD group.