According to family systems and life course theories, periods of intense change, such as early adolescence, can disrupt stable family systems, leading to changes in family relationships. In this longitudinal study, we investigate 2 types of change in parental hostility and warmth toward their children during early adolescence (Grades 6 to 8)--developmental trends (linear declines) and lability (within-person fluctuations around developmental trends)--and their linkages to youth substance use and delinquency in Grade 9 (N = 618). We also test if the linkages between lability and youth risky behavior are moderated by youth gender. After controlling for between-person differences in level and developmental trends, we find greater lability (more fluctuations) in youth-reported parents' warmth and hostility are associated with greater youth delinquency, tobacco use, and polysubstance use initiation. The associations between youth-reported lability in mother and father hostility and polysubstance use demonstrated an inverted U shape pattern: Moderate levels of lability were associated with higher substance use but very low and high lability was associated with relatively lower rates of substance use. Many of the linkages between lability and youth delinquency were significant for girls but not boys. Fewer effects of lability on youth outcomes were found using parent reports. Developmental trends in parents' warmth and hostility were also associated with youth delinquency. Lability has unique implications for youth adjustment, yet appears to differ by youth outcome, gender, and reporter. The discussion focuses on mechanisms that might link changes in parent-youth warmth and hostility to youth risky behavior.