Piaget＇s theory of psychological development can contribute to a better understanding of ADHD （attention deficit hyperactivity disorder）. The assumption is that children with ADHD present deficits in the development of the construction of operational notions, such as space-time conceptions and causality. Sixty-nine children （aged 6 to 12） were divided into two groups： children with ADHD and children without diagnosis. According to Piaget＇s clinical interview, Piagetian tasks were applied. The clinical sample was sub-divided into children who used methylphenidate and non-medicated children. ADHD children showed a tendency to response and reasoning considered most common at developmental levels inferior to the comparison group. Regarding the use of methylphenidate, there was no significant difference of performance between the subgroups. Results may be indicative that the deficits relate mainly to structuring aspects of thinking, and not to performance. Theoretical and practical implications were discussed.