This study examined whether the quality of relationships with foster caregivers was associated with autonomic nervous system reactivity of children during separation and reunion with their foster caregiver. Moreover, effects of early adversity were examined in relation to attachment and autonomic nervous system reactivity. The sample included 60 children between 26 and 88 months of age, who participated with their primary foster caregivers in the Strange Situation. Respiratory sinus arrhythmia and preejection period were measured as indicators of parasympathetic and sympathetic nervous system reactivity, respectively. Attachment quality (ordered/disordered and secure/insecure attachment), was coded on the basis of children's behavior in the Strange Situation using the Cassidy and Marvin coding system. Children with a background of neglect and those with disordered (disorganized–controlling or insecure–other) attachment showed most sympathetic reactivity during the procedure. Moreover, children with disordered attachment showed less vagal regulation (respiratory sinus arrhythmia decreases on separation and increases on reunion) than children with ordered attachment. The findings show that the quality of relationships with current caregivers, and to a lesser extent specific experiences of neglect, may have an impact on children's abilities to regulate emotions in the context of environmental stress and challenges.