Few longitudinal studies examine how "changes" in parent-child relationships are associated with changes in youth internalizing problems. In this longitudinal study, we investigated how developmental trends (linear change) and year-to-year lability (within-person fluctuations) in parental warmth and hostility across Grades 6 to 8 predict youth internalizing problems in Grade 9 (N = 618) and whether these linkages differ for boys and girls. Developmental trends (greater decreases in warmth and increases in hostility) were associated with more youth internalizing problems. Greater year-to-year lability (more fluctuations) in father hostility and warmth were also associated with more internalizing problems. Greater lability in mother warmth was associated with more internalizing problems for girls only. The strongest effects of lability on internalizing problems were found for youth with the highest lability scores. This study underscores the importance of differentiating developmental trends from lability in parent-child relationships, both of which may be important for youth internalizing problems.