Research Findings: The transition to school is a major developmental milestone, and behavior tendencies already evident at the point of school entry can impact upon a child's subsequent social and academic adjustment. The current study aimed to investigate stability and change in the social behavior of girls and boys across the transition from day care to 1st grade. Teacher ratings and peer nominations for prosocial and antisocial behavior were obtained for 248 children belonging to 2 cohorts: school transitioning (n = 118) and day care remaining (n = 130). Data were gathered again from all children 1 year later, following the older group's entry into school. Teacher ratings of prosocial and antisocial behavior significantly predicted teacher ratings of the same behavior at Time 2 for both cohorts. Peer reports of antisocial behavior also showed significant stability, whereas stability of peer-reported prosocial behavior varied as a function of behavior type. Practice or Policy: The results contribute to understanding of trends in early childhood social behavior that potentially influence long-term developmental trajectories. Identification of some behaviors as more stable in early childhood than others, regardless of school entry, provides useful information for both the type and timing of early interventions. (Contains 1 footnote and 2 tables.)