This study examined whether children with attention-deficit/hyperactivity disorder (ADHD) have lower autonomic nervous system (ANS) activity and show less stress reactivity than children with an anxiety disorder. It also explored whether such a difference was accounted for by comorbid oppositional defiant disorder (ODD) or conduct disorder (CD) in some of the ADHD children. Forty-three referred children performed a stress task, during which skin conductance (SCL) and heart rate (HR) levels were measured. Results showed that the ADHD group had similar SCL responses as the anxiety disorder group, but showed less HR reactivity immediately after the stress task. The ADHD with ODD/CD group had a slightly higher HR level than the pure ADHD group, but showed similar SCL and HR reactivity and recovery. It was concluded that ADHD children have less HR reactivity immediately after stress than children with an anxiety disorder, which was not accounted for by comorbid ODD/CD symptoms, and which may be related to a stronger parasympathetic than sympathetic activation.