Positive parent-child relationship quality is critical for buffering children from the effects of stress on development. It is thus vital to develop interventions that target parent-child relationship quality for families experiencing stress. We examined the moderating role of parent-child relationship quality (as measured by parental emotional availability [EA]) in the intergenerational association between parental adverse childhood experiences (ACEs) and their young children's hair cortisol concentrations (HCCs)--a physiological marker of cumulative hypothalamic pituitary adrenal (HPA)-axis activity. Using data from 127 parent-child dyads collected by two of six ACF-funded Buffering Toxic Stress consortium sites, we tested interaction effects of parental ACEs with parental EA on young children's (M[subscript age] = 18.38, SD[subscript age] = 7.10) HCC. Results revealed curvilinear main effects such that higher parental ACEs were significantly associated with greater HCC and stronger associations occurred at higher levels of parental ACEs. However, this association was moderated by parental EA. Thus, among children with higher parental history of ACEs, children of parents with higher EA had lower HCC compared to children of parents with lower EA. These findings provide support for the risk-buffering and risk-exacerbating role of parent-child relationship quality (e.g., EA) for the transmission of parents' early life adversity on their children's HPA-axis activity, documented here in a racially and ethnically diverse sample of children and parents served by Early Head Start. Findings suggest that intervention and prevention efforts targeting stress response in children of mothers with childhood adversity should also support parents in building an emotionally available relationship with their children.