Both longer term developmental changes (increases in hostility, decreases in warmth) and lability (year-to-year fluctuations) in parent-child relationship quality across childhood and adolescence have been linked to adolescent externalizing behaviors. Using a prospective longitudinal study of 561 children who were adopted into nonrelative families at birth (57% male, 56% White, 19% multiracial, 13% Black, 11% Hispanic) where parental warmth and hostility reflect environmental influences or child-evoked reactions, we examined associations between parent-child relationship measures and externalizing behaviors at age 11 and across adolescence (i.e., from age 11 to 13-15 years). Because studies considering gene-environment interplay especially in associations between lability and child externalizing behaviors are sparse and parent-child relationship measures support the intergenerational transmission of psychopathology, we also tested whether parent psychopathology of both adoptive parent (AP; environmental intergenerational transmission) and birth parents (genetic intergenerational transmission) moderated these associations in multivariate regression models. Findings generally supported more effects of fathers' than mothers' warmth and hostility. Although there were some linear associations of increased lability with externalizing behaviors, these did not persist in the context of a multivariate model. Associations between both parents' increasing hostility across childhood on age 11 externalizing behaviors and for fathers increasing hostility and decreasing warmth on increases in externalizing behaviors across adolescence more likely reflect a combination of bidirectional evocative and parenting environmental associations than purely parenting environmental transmission. Moderation by parent psychopathology was sparse, and sensitivity tests revealed no differences by child sex. (PsycInfo Database Record (c) 2024 APA, all rights reserved).