The present study used a longitudinal randomized clinical trial to test whether an early intervention has causal effects on children's autonomic nervous system regulation. When children were infants, parents involved with Child Protective Services received Attachment and Biobehavioral Catch-up (ABC; N = 43), an intervention that promotes sensitive parenting, or a control intervention (N = 53). When children were 9 years old, children whose parents had received ABC exhibited higher respiratory sinus arrhythmia and lower heart rate at rest and during a parent-child interaction than children in the control group. Intervention effects were not detected for children's average skin conductance levels or for indices of autonomic reactivity. Results suggest that a parenting-focused early intervention impacted the development of children's autonomic regulation.