In online discussion forums, many students' posts never receive a response, but it remains unclear whether this makes them feel ignored and causes emotional pain, negative social perceptions, and threatens fundamental psychological needs. Two studies support this prediction. Using an observational design, Study 1 showed that when students felt excluded during an online cooperative assignment, they also reported thwarted psychological needs (belonging, selfesteem, meaningful existence), more negative mood and social perceptions, decreased cooperation, and increased individualism. And using an experimental design, Study 2 demonstrated that excluding students' in an online discussion caused these outcomes. With implications for theory and research, findings suggest that feeling ostracized in online discussions is damaging and may undermine the positive effects of cooperation.