We recently reported that a randomized controlled trial of a family-focused intervention for parentally bereaved youth predicted higher cortisol output 6 years later relative to a control group of bereaved youth (Luecken et al., Psychoneuroendocrinology 35, 785–789, 2010). The current study evaluated longitudinal mediators of the intervention effect on cortisol 6 years later. Parentally bereaved children (N=139; mean age, 11.4; SD=2.4; age range=8–16 years; male; 61 % Caucasian, 17 % Hispanic, 7 % African American, and 15 % other ethnicities) were randomly assigned to the 12-week preventive intervention (n=78) or a self-study control (n=61) condition. Six years later (mean age, 17.5; SD, 2.4), cortisol was sampled as youth participated in a parent–child conflict interaction task. Using four waves of data across the 6 years, longitudinal mediators of the program impact on cortisol were evaluated. Program-induced increases in positive parenting, decreases in child exposure to negative life events, and lower externalizing symptoms significantly mediated the intervention effect on cortisol 6 years later.