This study examined the relationships between the quality of parent-child interactions, specifically interactional synchrony (IS), and physical and relational aggression in young children. Seventy-three children (3-6 years; 44 males, 29 females) and their mothers participated in this study. The children's level of aggression was assessed through questionnaires completed by their mothers and teachers/caregivers. Mother-child dyads engaged in a free play task and a structured task that were coded for IS. The level of IS only in the free play task, but not in the structured block task predicted children's parent-rated physical aggression levels, with lower levels of IS predicting more aggression. No significant sex differences were found in the levels of IS, physical or relational aggression. The findings are helpful for better understanding of parent--child relationships and problem behaviours and for making intervention recommendations. (Contains 5 tables.)