The course and malleability of aggressive behavior from beginning elementary school through transition into middle school was investigated. In a developmental epidemiologically defined population of 1000 urban first graders, a two year classroom-based randomized preventive trial was aimed at reducing aggressive behavior, an antecedent of delinquency, violent behavior, and heavy drug use in adolescence and adulthood. Earlier we reported impact in first grade on teacher and peer ratings and on classroom observations. We report here on the course and on sixth grade teacher ratings of aggressive behavior. Improvement was observable during transition times, in first grade and in middle school, among the males in the preventive intervention who were more aggressive in first grade.