We examined the longitudinal stability of measures of negative peer status and aggressive-disruptive behavior in preschool boys. Subjects were 53 white 4- to 5-year-old boys from low-income family backgrounds. Peer sociometric measures of rejection and behavioral deviance were assessed in the fall and spring of the preschool year. Complementary measures were also obtained from teachers at both assessment points. Half of the boys designated as rejected on the basis of peer nominations maintained this status at the end of the preschool year. Teachers and peers did not agree on their selections of socially rejected children, but had good agreement concerning the identification of children with externalizing-type behavior problems. Finally, teacher and peer classifications of aggressive-disruptive children were highly stable throughout the preschool year. These findings indicate that peer-rejected children can be identified at very young ages, and that preschoolers can be reliable informants about the social maladjustment of peers.