This study extends the growing literature of family instability by investigating the significance of its timing for children's social adjustment. We find that more than a third of the 1,364 elementary school children in the NICHD Study of Early Child Care and Youth Development experienced some family structure change. Their cumulative level of family instability was associated with multiple indicators of social development at the end of elementary school, associations that were especially strong for boys. These associations were driven largely by the lasting effect of family change in early childhood, demonstrating the significance of early family instability for children's later social development.