Background: Growing research on children's traits as moderators of links between parenting and developmental outcomes has shown that variations in positivity, warmth, or responsiveness in parent-child relationships are particularly consequential for temperamentally difficult or biologically vulnerable children. But very few studies have addressed the moderating role of children's callous-unemotional (CU) traits, a known serious risk factor for antisocial cascades. We examined children's CU traits as moderators of links between parent-child Mutually Responsive Orientation (MRO) and shared positive affect and future externalizing behavior problems. Methods: Participants included 100 two-parent community families of normally developing children, followed longitudinally. MRO and shared positive affect in mother-child and father-child dyads were observed in lengthy, diverse naturalistic contexts when children were 38 and 52 months. Both parents rated children's CU traits at 67 months and their externalizing behavior problems (Oppositional Defiant Disorder and Conduct Disorder) at 67, 80, and 100 months. Results: Children's CU traits moderated links between early positive parent-child relationships and children's future externalizing behavior problems, even after controlling for strong continuity of those problems. For children with elevated CU traits, higher mother-child MRO and father-child shared positive affect predicted a decrease in mother-reported future behavior problems. There were no significant associations for children with relatively lower CU scores. Conclusions: Positive qualities for early relationships, potentially different for mother-child and father-child dyads, can serve as potent factors that decrease probability of antisocial developmental cascades for children who are at risk due to elevated CU traits.