We tested three competing models regarding the role of deviant friends in the trajectorylinking early disruptiveness with later conduct problems through the use of a preventiveintervention program. The program was implemented during the second and third grade. Onemodel predicted that the program would positively affect later conduct problems by facilitatingnondeviant peer association during early adolescence. The second model predicted a directimpact of the program on later conduct problems through the reduction of early disruptiveness.The third model predicted an interaction between postintervention disruptiveness and associationwith less deviant friends. The results showed that the program's effects on later conductproblems were mediated by the reduction in disruptiveness and by the association with lessdeviant friends. However, the positive effect of associating with less deviant friends depended onwhether children's disruptiveness had been reduced or not by their participation in theprogram, thus supporting the third model. We recommend using intervention studies to testdevelopmental models.