Heritability of deficit and hyperactivity disorder (ADHD) is high. Thus, frequently both children and parents are affected. Parental ADHD often has a negative impact on parent-child interactions and may constitute a significant barrier to a successful treatment of the child's ADHD. The objective of our randomized controlled multicentre trial is to evaluate whether the treatment of maternal ADHD improves the efficacy of a behavioural parent training for children's ADHD. One hundred and forty-four mother-child pairs, both affected by ADHD according to DSM-IV, will be treated at five sites in Germany. Mothers are randomized to manualized cognitive-behavioural group psychotherapy plus open methylphenidate treatment or to control treatment (supportive counselling). After 13 weeks of treatment, manualized behavioural parent training will be administered to all mother-child pairs. Therapists are graduated psychologists or physicians. Treatment integrity will be established by independent supervision. Primary endpoint (child's externalizing symptoms) is rated by interviewers blind to the mother's treatment allocation. Intention-to-treat analysis will be performed within a linear regression model (Current Controlled Trials ISRCTN73911400).