This study examined sibling-dyad structural variables (sex composition, age difference, current coresidence, position adjacency, family size, respondent and/or sibling ordinal position) and attitudes toward adult sibling relationships. A sample of 1,053 young adults (M age = 22.1 years) described one sibling using the Lifespan Sibling Relationship Scale. Position adjacency and family size were related to attitudes toward sibling relationships, including more positive recalled childhood sibling relationships in adjacent dyads and larger families and less positive sibling relationships recalled from childhood and in adulthood experienced by individuals with only one sibling. Results for respondent and sibling ordinal position were consistent, with individuals in the youngest-of-two ordinal position and those describing eldest siblings reporting less positive attitudes toward adult siblings. Implications for future research on the quality of adult sibling relationships throughout the life span are discussed. (Contains 3 tables and 6 notes.)