Abstract. Objective: We examined predictors and moderators of treatment outcome in mothers and children diagnosed with ADHD in a large multicentre RCT. Method: In total, 144 mother-child dyads with ADHD were randomly assigned to either a maternal ADHD treatment (group psychotherapy and open methylphenidate medication, TG) or to a control treatment (individual counselling without psycho- or pharmacotherapy, CG). After maternal ADHD treatment, parent-child training (PCT) for all mother-child dyads was added. The final analysis set was based on 123 dyads with completed primary outcome assessments (TG: n = 67, CG: n = 56). The primary outcome was the change in each child’s externalizing symptoms. Multiple linear regression analyses were performed. Results: The severity of the child’s externalizing problem behaviour in the family at baseline predicted more externalizing symptoms in the child after PCT, independent of maternal treatment. When mothers had a comorbid depression, TG children showed more externalizing symptoms after PCT than CG children of depressive mothers. No differences between the treatment arms were seen in the mothers without comorbid depression. Conclusions: Severely impaired mothers with ADHD and depressive disorder are likely to need additional disorder-specific treatment for their comorbid psychiatric disorders to effectively transfer the contents of the PCT to the home situation (CCTISRCTN73911400).