This study examined correlates of attachment at age 3 to further validate preschool separation-reunion measures. Three-year-olds (N = 150) and their mothers participated in a separation-reunion protocol, the Preschool Attachment Classification System (PACS: J. Cassidy & R. S. Marvin with the MacArthur Working Group on Attachment, 1992), and a mother-child interaction session during a laboratory visit. Mothers also completed psychosocial measures and, along with teachers, evaluated child behavior problems. The secure and disorganized groups received, respectively, the highest and lowest interaction scores. Disorganized children showed a higher level of teacher-reported externalizing and internalizing problems than did secure children. Mothers of insecure children reported higher child externalizing (all insecure groups) and internalizing (avoidant group) scores, more personal distress related to emotional bonding (disorganized group), childrearing control (ambivalent group), and child hyperactivity (avoidant group). Results strongly support the validity of the PACS as a measure of attachment in 3-year-olds.