Research has documented that it is common for adult children to perceive that their parents favor some offspring over others in the family. Further, such perceptions have been shown to have negative consequences on sibling relationships in adulthood. However, little is known about patterns or consequences of stability in adult children's perceptions of maternal favoritism across time. In this paper, we address these questions, using data collected from 469 adult children nested within 216 families as part of the Within-Family Differences Study-I and -II. Comparison of adult children's reports at T1 and T2 showed that perceptions of maternal differential treatment (MDT) were remarkably stable over the 7-year period of the study, with more than three-quarters of the respondents reporting the same perceptions at both waves. Consistent with theories of equity and cumulative exposure to stress, multinomial regression analyses revealed that children who perceived MDT at only one time point reported higher sibling closeness at T2 than did those who perceived MDT at both T1 and T2, regardless of the particular pattern of changes in perceived MDT.