Objective: Sexually or physically abused children are at risk for neurobiological dysregulation as well as for internalizing and disruptive behavior disorders. Stress-related autonomic nervous system (ANS) down-regulation has been proposed as a sequela of abuse and was investigated in the present study. Methods: Child Protective Services documented incidents of abuse were recorded for children in a sample of 262 pediatric psychiatric inpatients, as well as demographic, physical and intellectual functioning, and diagnostic and medication prescription data. Before and after a mildly stressful blood draw, noninvasive assessments of ANS activity were obtained. Results: Controlling for all other variables in logistic regression analyses, a history of physical abuse (45% overall prevalence) was associated with poststressor ANS hyporesponsivity (i.e., heart rate deceleration). Conclusions: Results suggest that a history of physical (but not sexual) abuse is associated with stressor-related ANS down-regulation in psychiatrically impaired children and adolescents. Practice implications: Stressor-related autonomic hyporesponsivity secondary to physical abuse may contribute to the impairment of severely emotionally disturbed children. Differential diagnosis of psychiatrically impaired children should include identification of those who have a history of physical abuse, and their treatment should address stressor-related hyporeactivity. (Contains 4 tables.)