The degree to which parent antisocial characteristics moderated the effects of the Oregon model of Parent Management Training (PMTO™) on observed parenting practices over 2 years after baseline was assessed in a sample of recently married biological mother and stepfather couples with at-risk children. Sixty-seven of the 110 participating families were randomly assigned to PMTO, and 43 families to a non-intervention condition. Using an intent-to-treat analysis, PMTO was reliably related to growth in positive parenting and to decreases in coercive parenting. Parent antisocial characteristics moderated the effect of PMTO on coercive but not on positive parenting practices. PMTO resulted in greater reductions in coercive parenting as parent antisocial histories were more extensive, and this moderator effect was found for both mothers and stepfathers. The findings support the effectiveness of PMTO as a preventive intervention for child conduct problems, and indicate that the parenting behaviors of antisocial parents are malleable and serve as important mediators of their impact on child conduct problems.