This study examined relations among mothers' hostile attribution tendencies regarding their children's ambiguous problem behaviors, mothers' harsh discipline practices, and children's externalizing behavior problems. A community sample of 277 families (19% minority representation) living in three geographic regions of the United States was followed for over 4 years. Mothers' hostile attribution tendencies were assessed during the summer prior to children's entry into kindergarten through their responses to written vignettes. Mothers' harsh discipline practices were assessed concurrently through ratings by interviewers and reports by spouses. Children's externalizing behavior problems were assessed concurrently through written questionnaires by mothers and fathers and in the spring of kindergarten and first, second, and third grades through reports by teachers and peer sociometric nominations. Results of structural equations models demonstrated that mothers' hostile attribution tendencies predicted children's future externalizing behavior problems at school and that a large proportion of this relation was mediated by mothers' harsh discipline practices. These results remained virtually unchanged when controlling for initial levels of children's prekindergarten externalizing behavior problems at home.