Background: Screen-related behaviours are highly prevalent in schoolchildren. Considering the adverse health effects and the relation of obesity and screen time in childhood, efforts to affect screen use in children are warranted. Parents have been identified as an important influence on children's screen time and therefore should be involved in prevention programmes. The aim was to examine the mediating role of family-related factors on the effects of the school-based family-focused UP4FUN intervention aimed at screen time in 10- to 12-year-old European children (n child--parent dyads = 1940). Methods: A randomised controlled trial was conducted to test the six-week UP4FUN intervention in 10- to 12-year-old children and one of their parents in five European countries in 2011 (n child--parent dyads = 1940). Self-reported data of children were used to assess their TV and computer/game console time per day, and parents reported their physical activity, screen time and family-related factors associated with screen behaviours (availability, permissiveness, monitoring, negotiation, rules, avoiding negative role modeling, and frequency of physically active family excursions). Mediation analyses were performed using multi-level regression analyses (child-school-country). Results: Almost all TV-specific and half of the computer-specific family-related factors were associated with children's screen time. However, the measured family-related factors did not mediate intervention effects on children's TV and computer/game console use, because the intervention was not successful in changing these family-related factors. Conclusion: Future screen-related interventions should aim to effectively target the home environment and parents' practices related to children's use of TV and computers to decrease children's screen time. Trial registration: The study is registered in the International Standard Randomised Controlled Trial Number Register (registration number: ISRCTN34562078).