Vagal tone is proposed as a novel index of stress vulnerability and reactivity with applications in all branches of medicine, and with particular value in pediatrics. The paper proposes a model emphasizing the role of the parasympathetic nervous system and particularly the vagus nerve in mediating homeostasis and defining stress. Measurement of cardiac vagal tone is proposed as a method to assess on an individual basis both the stress response and the vulnerability to stress. The method monitors the neural control of the heart via the vagus (ie, vagal tone) as an index of homeostasis. The method provides a standard instrument with statistical parameters that are comparable between patients and throughout the life span. This noninvasive method will allow the assessment of the stressful impact of various clinical treatments on the young infant and permit the identification of individuals with vulnerabilities to stress.