This study examined the degree to which parent behavior and sibling behavior are differentially related to adolescent adjustment. Using reports from adolescent sibling pairs (41 seventh graders and an older sibling), this study was designed to examine the degree to which parent and sibling behavior (i.e., warm/supportive and hostile) uniquely, and in combination, were related to adolescents' academic achievement, problem behavior, and association with delinquent friends. Participants provided self-reports of their own, their parents', and their siblings' behavior, their own and their friends' problem behavior, and their own academic achievement. For both seventh graders and their siblings, parents' behavior predicted more variability in problem behavior and association with delinquent friends than it did in academic achievement. Across models, fathers' hostile behavior emerged as a significant predictor of both problem behavior and friends' problem behavior. Siblings' hostile behavior was a significant predictor of academic achievement for both groups of adolescents; however, it was a significant predictor of problem behavior and association with delinquent friends only for the older siblings. For the older siblings, the seventh graders' warm/supportive behavior predicted academic achievement. When parents' and siblings' behavior were considered in combination, seventh graders' and fathers' hostile behavior emerged as significant, unique predictors of the older siblings problem behavior. Moreover, the seventh graders' warm/supportive and hostile behaviors emerged as significant, unique predictors of the older siblings' academic adjustment. (Author)