Background: Delay aversion, the motivation to escape or avoid delay, results in preference for small immediate over large delayed rewards. Delay aversion has been proposed as one distinctive psychological process that may underlie the behavioural symptoms and cognitive deficits of attention deficit/hyperactivity disorder (ADHD). Furthermore, the delay aversion hypothesis predicts that ADHD children's preference for immediate small over large delayed rewards will be reduced when stimulation, which makes time appear to pass more quickly, is added to the delay interval. The current paper tests these predictions. Methods: A group of children with a diagnosis of ADHD (with or without oppositional defiant disorder (ODD)), a group with a diagnosis of high-functioning autism (HFA), and a normal control group were compared on an experimental paradigm giving repeated choices between small immediate and large delayed rewards (Maudsley Index of Delay Aversion-MIDA) under two conditions (stimulation and no stimulation). Results: As predicted, ADHD children displayed a stronger preference than the HFA and control children for the small immediate rewards under the no-stimulation condition. The ADHD children preferences were normalised under the stimulation condition with no differences between the groups. This pattern of results was the same whether the ADHD children had comorbid ODD or not. Discussion: The findings from the MIDA are consistent with the delay aversion hypothesis of ADHD in showing that preference for small immediate rewards over large delayed rewards is a specific feature of ADHD and that this preference can be reduced by the addition of stimulation. Further research is required to better understand the emotional and motivational mechanisms underpinning delay aversion.