This study using PROSPER data (N = 977, age 11.5-age 15) investigated the longitudinal within-family associations between parent reported parental monitoring and adolescent aggression. Importantly, this study is the first one to examine parent gender and adolescent gender differences on these within-family associations. Results differed between mothers and fathers. There was a negative, bidirectional within-family association between maternal monitoring and adolescent aggression, such that more maternal monitoring than usual was associated with fewer adolescent aggressive behavior problems than usual within the same family, and vice versa. In contrast, during mid-adolescence, a positive, bidirectional within-family association between paternal monitoring and adolescent males' aggression was found, such that more paternal monitoring than usual was related to more adolescent males' aggression than usual within the same family, and vice versa. Practical implications on intervention strategies are discussed.