Grounded in the family systems theory within the life course systems perspective, this study sought to investigate (a) long-term transmission processes of hostility from interparental relationships to parent-young adult relationships and (b) the mediational roles of parents' and adolescents' psychopathology in these long-term processes. Research has examined the long-term transmission of hostility from interparental relationships to young adults' relationships with their parents. However, less is known about how this transmission process differs for father-young adult and mother-young adult relationships and if psychopathology is a mechanism that contributes to the transmission of hostility. Utilizing a structural equation model and prospective, longitudinal data from couples in enduring marriages and their offspring (n = 345 families), we examined long-term associations between interparental hostility when offspring were adolescents and young adults (1990 and 2001), adolescents' hostile relationships with their fathers and mothers (1991), and young adults' hostile relationships with their fathers and mothers (2003) as well as the mediating roles of fathers', mothers', and adolescents' psychopathology (1992 and 1994). Interparental hostility was more strongly linked to hostility between mothers (rather than fathers) and their young adult offspring. Fathers', mothers', and adolescents' psychopathology uniquely mediated the transmission processes. This study elucidated transmission processes of interparental hostility to young adults' relationship hostility with their parents and offers insights into the unique mediational roles of each family member's psychopathology. (PsycInfo Database Record (c) 2023 APA, all rights reserved).