ADHD seriously impacts family functioning, even the more in families with simultaneous parental and child ADHD. The aim of the study was to examine associations between family impact of child ADHD and child, mother and family characteristics in multiplex families with children and mothers both affected by ADHD. One hundred and forty-four mother-child pairs were assessed (children: mean age 9.4 ± 1.7 years, 73.6 % male). Family impact of child ADHD was rated by mothers using the Family Impact Questionnaire (FIQ). Multiple linear regression analyses were performed with child and maternal psychopathology and basic family characteristics such as employment, partnership status and number of children as predictors and FIQ subscores as criteria. Rates of variance explained by family variables were 49 % for negative feelings towards the child, 37 % for impact on partnership, 31 % for impact on social life and 27 % for impact on finances (p < .001, respectively). Pearson correlations with family impact were especially strong for child externalizing symptoms, maternal ADHD and co-morbid symptoms of the mother. The strongest independent predictor was oppositional defiant child behaviour. In ADHD multiplex families, mothers' perception of the impact of an ADHD child on its family can be explained to a substantial degree by child psychopathology, maternal psychopathology and basic family characteristics. Although a cross-sectional design does not allow for causal interpretations, the findings of this study offer important targets for the treatment of ADHD in a family context pointing to the need for assessing and treating parental mental health and co-morbid symptoms besides ADHD core symptoms.