Objective: The present article tests the hypothesis of a sustained attention deficit in children and adults suffering from ADHD. Method: Vigilance and sustained attention of 52 children with ADHD and 38 adults with ADHD were assessed using a computerized vigilance task. Furthermore, the attentional performance of healthy children (N = 52) and healthy adults (N = 38) was examined. Results: Children and adults with ADHD performed significantly less well in the vigilance task than healthy participants (main effect for group). Furthermore, children and adults showed a significant decrease of performance over time (time-on-task effects). However, there was no greater decrement of performance with the passage of time in patient groups than in control groups (group-by-time interaction). Conclusion: The present results do not support the hypothesis of a sustained attention deficit in children and adults with ADHD.