This study examined the unique roles of peer rejection and affiliation with aggressive peers in the development of relational and physical aggression in a sample of 979 2nd through 4th grade children. Information about target children and their best friends' aggression and peer rejection was gathered via peer-nominations when the majority of children were in the 3rd grade, and again approximately one year later. Friendships were identified by having target children nominate their three best friends in their classroom. Path analyses conducted with children who had at least one reciprocated friendship revealed that peer rejection and friends' aggression predicted changes in target children's aggression; however, the patterns of relations varied by gender and form of aggression. Higher initial levels of rejection and friends' relational aggression predicted increases in relational aggression among girls only, whereas rejection and friends' physical aggression predicted increases in physical aggression among boys and girls. The significance of these results for the application of peer influence theories to relational aggression, and to females, is discussed.