Hyperkinetic disorders or attention-deficit/hyperactivity disorders (ADHD) are among the psychiatric diagnoses most often encountered in children and adolescents. Symptoms include inattention, hyperactivity and impulsivity. Within the multi-modal treatment system, the therapy with psychostimulants (e. g. methylphenidate) proved to be an effective in patients with ADHD. A double-blind trial is indicated in cases of unclear efficacy of methylphenidate at initial application or if psychostimulants have regularly been applied for more than one year with an leave-out trial delivering unequivocal results. Further it is helpful in verifying potential undesirable side effects and improving patients' compliance.