Parenting stress is influenced by parents' perceptions of their relationships with their children, which can vary widely for each parent depending on which child in the family is being considered. Because this within-parent variation is rarely studied, we investigated some of the differential perceptions that arise with respect to children's behaviour problems and parent-child negativity/positivity. Participants included a national sample of adoptive parents of 486 pairs of genetically unrelated siblings. Mothers reported different perceptions of their sibling children, and within the family, the child who was rated higher in behaviour problems also was regarded with more negativity and less positivity. Furthermore, the magnitude of the difference was uncorrelated with various indicators of the family and home environment, including parental education and occupational prestige, type of housing and crowding in the home, and progressive versus traditional childrearing attitudes. This pattern of results was consistent with three previously published sibling studies utilizing similar methods. The results point to the importance of examining child effects and within-parent differences in subjective aspects of the parenting experience. (Contains 4 tables and 1 note.)