The qualities of children's friendships that may promote the development of positive sibling relationships were examined. Thirty 3- to 5-year-old firstborn children whose families were expecting a 2nd child participated in longitudinal assessments of parent-child, sibling, and best-friend peer relationships. Families were studied from the last trimester of pregnancy to 14 months following the new child's birth. Peer relationship variables, particularly, the overall quality of peer play, engagement in fantasy play, and conflict management were found to be significant predictors of the quality of children's interactions with their 6- and 14-month-old siblings. Results are discussed in terms of the functions friendships may hold for helping children make the transition to the sibling role. This is a report of a longitudinal study of firstborn preschool children's transition to becoming a sibling. The purpose of the study was to examine the role of firstborn children's friendships in the adaptation to the newborn. It has become increasingly clear that the peer system can make important contributions to children's socialization and eventual adaptation (Parker & Asher, 1987). This report proposes that being a friend, which is a role children often have before becoming a brother or sister, may be valuable preparation for the sibling role. Recent research has focused on the adaptive properties of children's personal social networks (Berndt & Perry, 1986; Felner, 1984; Furman & Buhrmester, 1985; Lewis, Feiring, & Kotsonis, 1984; Reid, Landesman, Treder, & Jaccard, 1989). These studies have confirmed that peers are significant members of children's social networks for children in early and middle childhood. The opportunity to interact with peers has also been associated with enhanced social development. In the primate world, repeated exposures to socially normal peers has been associated with the remediation of abnormal social behaviors among infant rhesus monkeys reared under isolate conditions (Suomi & Harlow, 1972). Furman, Rahe, and Hartup (1979) have applied these findings to socially withdrawn 4- and 5-year-old children. When exposed to same-aged and younger peers, these children were significantly more sociable in posttreatment observations than those who were not exposed to peers.