Considerable research links chronic pain to autonomic nervous system (ANS) dysfunction, specifically low heart rate variability (HRV) mediated by reduced parasympathetic activity. However, little is known about factors that influence ANS function in chronic pain. The ANS is the primary pathway for brain–gut communication, making it of particular interest in gastrointestinal disorders, such as irritable bowel syndrome, characterized by functional abdominal pain (FAP). We evaluated the relation of sex, pain severity, and psychological stress to ANS function in adolescents/young adults from a database of pediatric FAP and control participants enrolled 8 years earlier in a prospective study of pain. At follow-up in adolescence/young adulthood (Mean age = 19.46, SD = 3.48), we classified participants as Pain-Remit (n = 130), Pain-Persist (n = 96), and pain-free controls (n = 123). We recorded electrocardiogram data at rest and during laboratory stressors. Results demonstrated significantly lower HRV in Pain-Persist females compared with Pain-Remit females, female controls, and all males regardless of pain category. Spectral analysis of electrocardiogram showed that Pain-Persist females had reduced power in the high frequency domain of cardiac activity, ie, reduced parasympathetic “braking” of sympathetic activity, both at rest and during stress. Pain-Remit females exhibited levels of autonomic imbalance intermediate between those of females with persistent FAP and all other participants. Parasympathetically mediated low HRV in young women with persistent FAP may reflect a peripheral mechanism (eg, gut dysfunction) or a central nervous system mechanism (eg, pain amplification or poor emotion self-regulation) involving prolonged sympathetic activation.