Background: It has been proposed that ADHD is an executive control disorder. Little is known however about the maturation of executive control in ADHD. Method: A cross-sectional study of ADHD patients compared to normal controls tested on a computerized neurocognitive test battery. Participants: 175 patients with ADHD, age 10 to 29, compared to 175 age-matched normal controls. Results: In every age group, ADHD patients were impaired in measures of psychomotor speed, reaction time, cognitive flexibility, and attention. Participants in both groups improved with age. In tests of executive control, normals improved their performance with shorter reaction times. ADHD patients improved their performance but by adopting a less efficient strategy: Their reaction times increased with age. Conclusions: These data support executive control as a "core deficit" in ADHD. In the Stroop and the shifting attention tests, ADHD patients proved to be inefficient in allocating their attentional resources. (Contains 3 tables and 5 figures.)