This paper is on the influences of the classroom context on the course andmalleability of aggressive behavior from entrance into first grade through the transition intomiddle school. Nineteen public elementary schools participated in developmentalepidemiologically based preventive trials in first and second grades, one of which was directed atreducing aggressive, disruptive behavior. At the start of first grade, schools and teachers wererandomly assigned to intervention or control conditions. Children within each school wereassigned sequentially to classrooms from alphabetized lists, followed by checking to insurebalanced assignment based on kindergarten behavior. Despite these procedures, by the end offirst quarter, classrooms within schools differed markedly in levels of aggressive behavior.Children were followed through sixth grade, where their aggressive behavior was rated bymiddle school teachers. Strong interactive effects were found on the risk of being highlyaggressive in middle school between the level of aggressive behavior in the first gradeclassrooms and each boy's own level of aggressive, disruptive behavior in first grade. Themore aggressive first grade boys who were in higher aggressive first grade classrooms were atmarkedly increased risk, compared both to the median first grade boys, and compared toaggressive males in lower aggressive first grade classrooms. Boys were already behaving moreaggressively than girls in first grade; and no similar classroom aggression effect was foundamong girls, although girls' own aggressive behavior did place them at increased risk. Thepreventive intervention effect, already reported elsewhere to reduce aggressive behavior amongthe more aggressive males, appeared to do so by reducing high levels of classroom aggression.First grade males' own poverty level was associated with higher risk of being moreaggressive, disruptive in first grade, and thereby increased their vulnerability to classroom levelof aggression. Both boys and girls in schools in poor communities were at increased risk of beinghighly aggressive in middle school regardless of their levels of aggressive behavior in first grade.These results are discussed in terms of life course/social field theory as applied to the role ofcontextual influences on the development and etiology of severe aggressive behavior.