Objective: How best to capture heterogeneity in attention-deficit/hyperactivity disorder (ADHD) using biomarkers has been elusive. This study evaluated whether emotion reactivity and regulation provide a means to achieve this. Method: Participants were classified into three groups: children with ADHD plus low prosocial behavior (hypothesized to be high in callous/unemotional traits; n = 21); children with ADHD with age-appropriate prosocial behavior (n = 54); and typically developing children (n = 75). Children completed a task with four conditions: negative induction, negative suppression, positive induction, and positive suppression of affect. The task required children to view an emotion-laden film clip, while either facially mimicking (induction) or masking (suppression) the emotion of the main character. Parasympathetic and sympathetic nervous system activity were assessed via respiratory sinus arrhythmia (RSA) and cardiac pre-ejection period (PEP), respectively. Symptoms of anxiety, conduct, and oppositional defiant disorders were treated as covariates. Results: The ADHD-typical-prosocial group displayed atypically elevated parasympathetic reactivity (emotion dysregulation) during positive induction, along with increased sympathetic activity (elevated arousal) across conditions. In contrast, the ADHD-low-prosocial group displayed reduced parasympathetic reactivity and reduced sympathetic activity (low emotional arousal) across baseline and task conditions. Thus, both ADHD groups had altered patterns of autonomic functioning, but in two distinct forms. Conclusion: Although ADHD is heterogeneous clinically, results suggest that ADHD is also heterogeneous with regard to physiological indices of emotion and regulation. Future studies of emotion, regulation, and ADHD should take this into account. Further study of physiological responding in ADHD may yield clinically and etiologically distinct domains or groups. (Contains 2 figures and 3 tables.)