The need-to-belong theory stipulates that social exclusion fosters aggression, whereas the social-reconnection hypothesis suggests that social exclusion promotes motivation to behave cooperatively. To date, empirical investigations of these contrasting views have focused on the immediate effects of social exclusion, yielding mixed results. Here we examine longer term effects of preschool social exclusion on children’s functioning 2 years later. Social exclusion was reported by teachers, aggression and cooperation by parents. Cross-lagged analyses showed that greater social exclusion at age 4 predicted more aggression and less cooperation at age 6, providing support for the need-to-belong rather than social-reconnection hypothesis. Secondary analyses showed that social exclusion predicted more aggression only among children scoring above mean on aggression at age 4, indicating that aggressive behavior is amplified by social exclusion among children already behaving aggressively. No gender differences were found. Implications and limitations are discussed in a developmental context.