The authors examined (a) the extent to which kindergarten estimates of core language functions predicted teacher ratings of behavior problems in each of the child's first 4 years of elementary school and (b) the ability of core language measures to predict concurrent behavior problems at each of the early elementary school grades studied. Participants were 74 African American children who were recruited as infants into a longitudinal study of children's health and development. Sixty percent of the families were classified as low-income when the children entered kindergarten. Conduct problems and hyperactivity were assessed with the Conners' Teacher Rating Scale, core language functions with the Clinical Evaluation of Language Fundamentals-3 (CELF-3) and Peabody Picture Vocabulary Test-Revised (PPVT-R), and verbal working memory with the Competing Language Processing Task (CLPT). Results indicated that expressive and receptive language at kindergarten predicted teacher ratings of conduct problems, with increasing accuracy as children moved from kindergarten to third grade, particularly for receptive language. None of the early language measures predicted hyperactivity at any of the grades. Concurrent relationships, expressive language, conduct problems, and hyperactivity were stronger in second grade than in kindergarten, while lower scores in working memory predicted higher teacher-reported hyperactivity. These findings underscore the importance of core language functions in the prediction of behavior problems. (Contains 5 tables.)