This article examined the efficacy of a primary-level, standard-protocol behavior intervention for students with externalizing behavioral disorders. Elementary schools were randomly assigned to treatment (behavior intervention) or control (business as usual) conditions, and K-3 students were screened for externalizing behavior risk status. The final sample included 7 treatment schools (n = 44 students) and 6 control schools (n = 26 students). Results of multilevel models showed that students with externalizing behavior in the treatment schools had significantly lower levels of problem behavior than those in the control schools. A positive but statistically nonsignificant treatment trend was observed for increased on-task behavior. No effects were observed for academic skills. The positive effects of the behavior intervention were smaller in schools serving higher proportions of students with low socioeconomic status and for students who had higher baseline levels of externalizing behavior. The discussion includes the results, practical importance, and limitations. (Contains 4 tables and 2 figures.)