As part of a longitudinal study examining sibling and friend relationships in early and middle childhood, relational aggression by 4-year-olds and their interaction partners in semi-structured free play sessions with siblings and friends was examined during sibling sessions involving both same-gender and mixed-gender sibling pairs. Identifiable acts of relational aggression occurred during many of the interactions observed, but there was also a wide range in the amount of relational aggression produced. Both boys and girls used relational aggression with their siblings much more than they did with their friends. Although boys' and girls' relationally aggressive behaviors occurred at similar rates, the form and function of their relational aggression varied depending on the age and gender of their sibling.