To investigate associations of family-related factors with children's breakfast consumption and BMI-z-score and to examine whether children's breakfast consumption mediates associations between family-related factors and children's BMI-z-score.Ten- to twelve-year-old children (n = 6374; mean age = 11.6 ± 0.7 years, 53.2% girls, mean BMI-z-score = 0.4 ± 1.2) and one of their parents (n = 6374; mean age = 41.4 ± 5.3 years, 82.7% female, mean BMI = 24.5 ± 4.2 kg/m(2)) were recruited from schools in eight European countries (Belgium, Greece, Hungary, the Netherlands, Norway, Slovenia, Spain, and Switzerland). The children self-reported their breakfast frequency per week. The body weight and height of the children were objectively measured. The parents responded to items on family factors related to breakfast (automaticity, availability, encouragement, paying attention, permissiveness, negotiating, communicating health beliefs, parental self-efficacy to address children's nagging, praising, and family breakfast frequency). Mediation analyses were performed using multi-level regression analyses (child-school-country).Three of the eleven family-related variables were significantly associated with children's BMI-z-score. The family breakfast frequency was negatively associated with the BMI-z-score; permissiveness concerning skipping breakfast and negotiating about breakfast were positively associated with the BMI-z-score. Children's breakfast consumption was found to be a mediator of the two associations. All family-related variables except for negotiating, praising and communicating health beliefs, were significantly associated with children's breakfast consumption.Future breakfast promotion and obesity prevention interventions should focus on family-related factors including the physical home environment and parenting practices. Nevertheless, more longitudinal research and intervention studies to support these findings between family-related factors and both children's breakfast consumption and BMI-z-score are needed.