Children's peer relations represent a key aspect of school adjustment. However, little is known about their social-cognitive precursors. To address this gap, the authors followed 70 children across the transition to primary school. At Time 1 (age 5), Time 2 (age 6), and Time 3 (age 7), children were assessed on their theory of mind, prosocial behavior, and verbal ability. In addition, at Time 2 and at Time 3, the authors gathered peer nominations. Results supported the authors' mediational hypothesis of indirect paths from early theory of mind to subsequently lower peer rejection and higher peer acceptance, via improvements in prosocial behavior. The authors discuss implications of these longitudinal effects for the understanding of the impact of social-cognitive achievements for children's developing social relations. (Contains 2 figures and 2 tables.)