Prior research has confirmed a casual path between social rejection and aggression, but there has been no clear explanation of why social rejection causes aggression. A series of experiments tested the hypothesis that social exclusion increases the inclination to perceive neutral information as hostile, which has implications for aggression. Compared to accepted and control participants, socially excluded participants were more likely to rate aggressive and ambiguous words as similar (Experiment 1a), to complete word fragments with aggressive words (Experiment 1b), and to rate the ambiguous actions of another person as hostile (Experiments 2-4). This hostile cognitive bias among excluded people was related to their aggressive treatment of others who were not involved in the exclusion experience (Experiments 2 and 3) and others with whom participants had no previous contact (Experiment 4). These findings provide a first step in resolving the mystery of why social exclusion produces aggression.