This report examines young adults' states of mind regarding their early attachment experiences in relation to the observed and perceived quality of their sibling relationships. Sixty sibling pairs (18-25 years of age) were (a) administered the Adult Attachment Interview (George, Kaplan, & Main, 1985), (b) videotaped during a conflict resolution task, and (c) asked to describe the quality of their relationship using the Adult Sibling Relationship Questionnaire (Stocker, Lanthier, & Furman, 1997). As hypothesized, dismissing states of mind were associated with lower levels of positive and negative affect while participants attempted to resolve an area of conflict with a sibling as well as with relatively low levels of reported warmth in the relationship. In contrast--but also in line with predictions--preoccupied states of mind were associated with heightened expression of negative affect toward a brother or sister, and the siblings of highly preoccupied individuals reported more conflict in their relationships. Findings provide further support for the importance of young adults' representations of childhood attachment experiences with respect to the quality of their adult relationships. In addition, this study extends previous findings regarding the significance of dismissing versus preoccupied states of mind by demonstrating that these dimensions are differentially associated with behavioral and self-reported aspects of sibling relationship quality in early adulthood. (Contains 2 tables.)