The authors examined siblings' dyadic and differential conflict frequency with mothers and fathers from 7 to 19 years of age. Participants were first- and second-borns from 201 families who reported their conflict with each parent in 4 home interviews spaced over 5 years. Multilevel models examining trajectories of conflict frequency across age and year of study revealed that (a) consistent with a spillover hypothesis, elevation in parent-offspring conflict frequency was timed to firstborns' transition to adolescence for both siblings; and (b) consistent with a learning-from-experience hypothesis, there was no increase in conflict frequency at second-borns' transition to adolescence. These findings highlight the importance of studying the development of parent-offspring conflict within the larger family system. Copyright (c) 2007 APA, all rights reserved.