Gender segregation in the classroom is advocated as academically beneficial, particularly for girls. However, the social impact for children has received little attention. The present study compared children's peer relations following the transition from mixed-sex fourth-grade classrooms to same-sex fifth-grade classrooms, and beyond into same-sex sixth-grade classrooms. Participants completed self and peer measures of social competence. The change to same-sex classrooms was associated with more mutual friendship nominations among boys in both fifth- and sixth-grades. Peer nominated overt and relational aggression, victimization, rejection, and passive/withdrawn behaviors increased for girls in fifth-grade, but decreased in sixth-grade. Implications for children's social development are discussed in view of research showing academic benefits of same-sex classrooms.