The present study compared the sensitivity to pre- and post-reward delays of children with and without DSM-IV combined type ADHD. Thirty-six children with DSM-IV combined type ADHD and 36 controls completed a signal-detection task. Correct identification of one stimulus produced an immediate reward and then a 3·5 s delay before the next trial (immediate reward). Correct identification of the other stimulus was associated with a 3·5 s delay before reward was delivered (delayed reward). Group differences emerged in response bias toward the immediately rewarded alternative. Children in the ADHD group showed a greater bias toward immediate reward than the controls. Children in the control group showed different patterns of response bias and discriminability following immediate and delayed rewards. For these children discriminability was lower and response bias was greater on trials following delayed reward. Both groups responded more slowly on trials following delayed reward. These findings support the hypothesis that children with ADHD are unusually sensitive to pre-reward delays.