Objective: Investigate (1) the association of child adiposity with parent-to-child and parent-to-parent hostility, (2) the mediation of these associations by dietary behaviours, and (3) moderation by gender. Design: One hundred thirty-five couples with 6- to 14-year-old children completed measures of emotional and physical aggression, overreactive discipline, and child diet. Parent-to-parent hostility was also coded from laboratory observations. Main Outcome Measure: Child adiposity was a combination of body mass index and waist-to-hip ratio. Results: Mother-to-child hostility was associated with child adiposity. This association was concentrated in boys and was not significantly explained by child dietary factors. Mother-to-father hostility was not significantly associated with boys’ or girls’ adiposity. Girls’ adiposity was not significantly associated with family hostility. Fathers’ hostility was not linked to child adiposity. Conclusion: This is the first study to take a family-level approach to understanding the relation of hostility to child adiposity by examining relations among adiposity and both mothers’ and fathers’ hostility directed toward one another and toward their children. Our findings highlight the potential role played by mothers’ emotional hostility in boys’ adiposity and suggest that, if this role is further substantiated, mother-son emotional hostility may be a promising target for the prevention of child obesity.