OBJECTIVE: Peer relationships are one of the important factors in children's development. The present study examines the relationship between the effects of early peer relationships and adolescent psychological adjustment. METHODS: The first survey took place from 1998 to 2000, and a follow-up assessment obtained data in 2006, as the original participants reached 13-15 years of age. The first assessment used the Korean version of the Child Behavior Checklist (K-CBCL) and simple questions about peer relationships to evaluate the participants. The follow-up assessment administered the Korean Youth Self Report (K-YSR). RESULTS: Children's peer relationships have longitudinal effects on mental health and adjustment. Children who had qualitative peer-relation problems were more likely to exhibit internalizing problems as adolescents. CONCLUSION: Children who have poor peer relationships might become more vulnerable to emotional problems and social adjustment as adolescents.