Research on adult sibling relationships suggests a concern about the quality of these relationships. The meaning of closeness in sibling relationships across the adult life span, its perceived origins, maintenance, and dynamics, were investigated in a sample of 55 adults ranging in age from 25 to 93 years. Semi-structured interviews with the subjects in small groups elicited perceptions of closeness to siblings, feelings of rivalry, perceptions of critical incidents and their consequences, and changes of feelings and interactions over time. Analyses revealed that most subjects perceived themselves as always having been close to their siblings, some as having grown more or less close over time, and a few as never having been close. Most commonly, closeness was perceived as a childhood development. A variety of factors led interactively to the development of closeness, contributed to its maintenance, and to its transformation of meaning over time. The social factors included norms, values and religious preferences of the culture; familial factors included child-rearing practices and family traditions; structural factors included the number of children in the family, their sex, age spacing, and birth order; personal-interactive factors included sharing of physical space, values, experiences, critical incidents, and developmental tasks; and personal factors included personal characteristics and special interests. The results suggest that sibling closeness differs from intimacy and gains stability from existing within the context of internalized values for family unity and sibling solidarity. (Author/NRB)